the energy source for the coastal trapped waves in the australian coastal experiment region abstract the sea level on the southern australian coast is examined for the source of the coastal trapped wave energy observed during the australian coastal experiment sea level adjusted for atmospheric pressure and atmospheric pressure are observed to propagate eastward at about 10 m s 1 at the lowest frequency examined 24 day period some energy travels south along the west coast of tasmania but does not reach the east coast of mainland australia while some energy travels through bass strait to reach the east coast of mainland australia at the most energetic frequency 8 day period adjusted sea levels are coherent over the 3700 km of coastline from southern australia to the east coast and much of the wind forced coastal trapped wave energy appears to travel through bass strait to the mainland east coast we have not identified a mechanism for energy transfer through bass strait and we do not know what fraction of the coastal trapped wave energy incident on western bass strait actually reach

integration of ice core marine and terrestrial records for the australian last glacial maximum and termination a contribution from the oz intimate group the degree to which southern hemisphere climatic changes during the end of the last glacial period and early holocene 30 8 ka were influenced or initiated by events occurring in the high latitudes of the northern hemisphere is a complex issue there is conflicting evidence for the degree of hemispheric teleconnection and an unresolved debate as to the principle forcing mechanism s the available hypotheses are difficult to test robustly however because the few detailed palaeoclimatic records in the southern hemisphere are widely dispersed and lack duplication here we present climatic and environmental reconstructions from across australia a key region of the southern hemisphere because of the range of environments it covers and the potentially important role regional atmospheric and oceanic controls play in global climate change we identify a general scheme of events for the end of the last glacial period and early holocene but a detailed reconstruction proved problematic significant progress in climate quantification and geochronological control is now urgently required to robustly investigate change through this period copyright c 2006 john wiley sons ltd

u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

thermo rheological shear heating model for leucogranite generation metamorphism and deformation during the proterozoic trans hudson orogeny black hills south dakota abstract this paper evaluates thermotectonic models for metamorphism and leucogranite generation during the proterozoic trans hudson orogeny as recorded in rocks exposed in the black hills sd intrusion of the harney peak granite and associated pegmatites at 1715 ma occurred at the waning stages of regional deformation and staurolite grade regional metamorphism published consortium for continental reflection profiling cocorp results indicate that proterozoic sedimentary rocks were thrust over the archean wyoming province during the trans hudson collision isotopic compositions of the harney peak granite suggest that the exposed proterozoic and archean metasedimentary rocks in the black hills represent source rocks of the granites numerical simulations of the regional metamorphism and harney peak granite generation assuming crustal thickening by thrusting coupled with erosion show the following 1 doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust 2 a 35 km drop off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level it is however insufficient to produce staurolite and melting can occur only in the deepest parts of the crust 3 temperatures in crust with stable 70 km thickness for 40 ma and 35 km drop off length for heat production could become sufficient to produce staurolite at the current erosion level and subsequent rapid denudation of the crust could potentially trigger decompression melting of lower crustal rocks although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation it is inconsistent with melting of upper crustal proterozoic source rocks that is indicated by isotopic compositions of the granites with lack of evidence for rapid denudation of the trans hudson orogen and with confinement of the leucogranites to the deformed proterozoic metapelitic rocks production of the harney peak granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the wyoming province and overthrusted sedimentary rocks we suggest that with reasonable rheologic properties of metapelites and rates of plate convergence shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust modeling rheology of the lithologically stratified thickened crust with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny which explains regional folding of the schists and predicts ductile shear zones in the granite source region because of the lithologic stratification the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists a weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens

coastal upwelling by wind driven forcing in jervis bay new south wales a numerical study for 2011 abstract the princeton ocean model pom was used to investigate an upwelling event in jervis bay new south wales se australia with varying wind directions and strengths the pom was adopted with a downscaling approach for the regional ocean model one way nested to a global ocean model the upwelling event was detected from the observed wind data and satellite sea surface temperature images the validated model reproduced the upwelling event showing the input of bottom cold water driven by wind to the bay its subsequent deflection to the south and its outcropping to the surface along the west and south coasts nevertheless the behavior of the bottom water that intruded into the bay varied with different wind directions and strengths upwelling favorable wind directions for flushing efficiency within the bay were ranked in the following order n 0 northerly nne 30 northeasterly nw 315 northwesterly ne 45 northeasterly ene 60 northeasterly increasing wind strengths also enhance cold water penetration and water exchange it was determined that wind driven downwelling within the bay which occurred with nne ne and ene winds played a key role in blocking the intrusion of the cold water upwelled through the bay entrance a northerly wind stress higher than 0 3 n m 2 was required for the cold water to reach the northern innermost bay

a modeling study of the climatological current field and the trajectories of upwelled particles in the east australian current the climatological current field off the coast of new south wales australia is investigated using results from a field experiment and a diagnostic numerical modeling study in particular the flow dynamics are examined near smoky cape 308559s in the vicinity of the east australian current separation point modeled velocity fields compare favorably with observed velocity measurements that were obtained in this region during november 1998 an investigation of the role of advection in the alongshore momentum equation reveals that uvx and vvy change sign about the separation point representing a shift in the flow regime from onshore advection and southward acceleration north of the separation to seaward advection and weaker southward currents south of the separation bottom stress increases on the continental shelf downstream of the separation point coincident with high horizontal divergence throughout the region south of smoky cape the near bottom flow is characterized by a low burger number coincident with a subcritical richardson number these characteristics imply that the timescale in which bottom boundary layer shutdown occurs will be lengthened enabling a persistent flow of potentially nutrient rich water into the coastal region a series of lagrangian particle tracking experiments designed to investigate the fate of the upwelled water are presented motivated by an observed chlorophyll a maximum along the 25 25 su isopycnal water of this density is used as a proxy for nutrient enriched water it is found that upwelled water originates at depth to the north of smoky cape and at times well offshore south of smoky cape coastal recirculations are evident suggesting that upwelled water could be trapped in eddy fields adjacent to the coast and thus explaining why phytoplankton blooms are prevalent in the region south of smoky cape

lu hf and pbsl geochronology of apatites from proterozoic terranes a first look at lu hf isotopic closure in metamorphic apatite abstract the mineral apatite is characterized by elevated and highly variable lu hf ratios that in some cases allow for single crystal dating by the lu hf isotopic system apatites from the adirondack lowlands and otter lake area in the grenville province and from the black hills south dakota yield lu hf ages that are consistently older than their respective pb step leaching ages isotopic closure for the lu hf system therefore occurs before u pb system closure in this mineral in the adirondack lowlands where h2o activity was low lu hf systematics of cm sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism 700 to 675 c at 1170 1130 ma the relatively old lu hf ages of 1270 and 1230 ma observed for these apatites correlate with decreasing crystal size in contrast apatite from the fluid rich otter lake area and black hills yields unrealistically low apparent lu hf closure temperatures implying that in these apatites fluids facilitated late exchange the lu hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size or by fluid activity

textural geochronological and chemical constraints from polygenetic titanite and monogenetic apatite from a mid crustal shear zone an integrated epma sims and tims study abstract titanites extracted from a 520 9 ma syn orogenic syenite at otjimbingwe damara orogen namibia show shape colour and u pb systematics that vary with position in the syenite relative to a major shear zone most titanite fractions from syenite cropping out outside the shear zone are characterized by a medium brown anhedral core overgrown by a pale brown subhedral rim empa and sims analyses show that the cores are enriched in al fe and sometimes na mg and mn but also in ree nb and zr relative to the rim tims u pb ages from these titanites gave relatively imprecise 207pb 206pb ages between 680 21 ma and 695 20 ma as well as 570 8 ma to 604 9 ma mainly due to their low 206pb 204pb ratios combined th u pb relationships suggest that these titanites contain a component similar to ca 715 750 ma old titanite populations observed so far only in neoproterozoic syenites from the northern part of the orogen titanites from syenites within the shear zone are dark brown mostly euhedral to subhedral and display only occasionally a pale brown rim these titanites show decreasing concentrations of na mg al and fe and increasing concentrations of mn from core to rim interpreted to reflect growth zonation abundances of mree hree y nb and zr are higher in the rim than in the core 207pb 206pb ages obtained on these titanites range from 470 2 ma to 538 3 ma for most samples only the samples with the highest 206pb 204pb ratios between 350 and 230 have u pb ages similar to the previously published u pb zircon age of 520 9 ma from the same complex other titanite fractions with lower 206pb 204pb ratios have slightly older ages than ca 520 ma indicating some inheritance inherited titanite survived multiple metamorphic episodes in the damara orogen between ca 750 ma and 470 ma and the igneous event that lead to the intrusion of the otjimbingwe syenite at 520 ma these data imply a high closure temperature of u pb in titanite of 650 700 c consistent with previous studies coeval apatite has no apparent inheritance low 206pb 204pb ratios between 55 and 170 and u pb ages similar to or only slightly younger than the u pb zircon or titanite age obtained on the titanite fractions with the highest 206pb 204pb ratios assuming a closure temperature for u pb of 900 c for zircon 700 c for titanite and 450 c for apatite initial fast cooling of the complex can be inferred our data demonstrate that titanite can contain multiple age domains preserving a chronological and chemical record of previous igneous events

concentric slow cooling of a low p high t terrane evidence from 1600 1300 ma mica dates in the 1780 1700 ma black hills orogen south dakota u s a the crystalline core of the southern black hills south dakota exposes an extensive low p high t aureole of garnet to second sillimanite zone schists centered on the plutonic core of the 1715 ma harney peak granite hpg this paper demonstrates regional patterns of apparent ages observed for 52 40 ar 39 ar dates of muscovite and biotite in diverse rocks from across the 1000 km 2 metamorphic aureole and its plutonic center about 20 biotite dates sampled mostly near faults are influenced by excess 40 ar and obscure the regional trends the remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite with previously unrecognized elliptical age zones centered on the main hpg pluton and its outliers the regional pattern of 40 ar 39 ar cooling ages indicates non uniform slow cooling of the mid crust between 1600 1250 ma this scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite rb sr and apatite u pb which range from 1690 to 1550 ma and from 1700 to 1500 ma respectively to explain these results it is likely that ambient pre granite temperatures of the country rocks were 350 c at the 10 14 km depth of granite emplacement as previously proposed and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years alternatively or in addition the hpg and inner aureole were not uplifted until 1480 1330 ma whereupon they finally cooled through 300 350 c

u pb ages on titanite and apatite from the darling range granite implications for late archaean history of the southwestern yilgarn craton abstract titanite and apatite u pb ages are reported for granites from the darling range batholith and a mafic granulite from the southwestern part of the archaean yilgarn craton of western australia the results provide constraints on the post emplacement temperature history of the batholith and the southwestern yilgarn craton the similarity of titanite and apatite ages at ca 2630 ma in the eastern part of the batholith and the adjacent metamorphic belt suggest that this region cooled from ca 650 c to 500 c within a time span represented by the uncertainties in the age measurements in the western part of the batholith titanite grains from one granite sample show two clearly resolved concordant ages of 2637 5 ma and 2615 3 ma and apatite ages from the granites are consistent at ca 2575 ma this is interpreted as evidence that the western part of the batholith cooled at a slower rate than the eastern part to explain this it is proposed that greater uplift in the west has exposed deeper granites than those exposed in the east this model is supported by gravity and seismic profiles across the southern yilgarn that suggest that the craton has been tilted with uplift being greater in the west westward uplift can also explain the progressive change in rb sr biotite ages across the batholith from ca 500 ma in the west to ca 2 5 ga in granites to the east the u pb systems of apatite from granites from the western part of the batholith have been slightly disturbed at ca 420 ma suggesting that in addition to uplift there was a reheating along the western margin of the craton possibly associated with continental rifting and movement on the darling fault

on the east australian current variability encroachment and upwelling 1 observations from an intensive oceanographic field program which took place in 1998 1999 about the separation point of the east australian current eac show significant spatial and temporal variability of the eac upstream of the separation point southward flowing currents are strong with subinertial velocities of up to 130 cm s 1 in the near surface waters whereas downstream currents are highly variable in both strength 1 70 cm s 1 and direction upwelling is observed to occur through both wind driven and current driven processes with wind effects playing a lesser role by contrast the encroachment of the eac upon the coast has a profound effect on the coastal waters accelerating the southward alongshore currents and decreasing the temperature in the bottom boundary layer bbl by up to 5 degreesc as the axis of the jet moves onshore negative vorticity increases in association with an increase in nonlinear acceleration during this time bottom friction is increased the burger number is reduced and the bbl shut down time lengthens the observed upwelling is attributed to enhanced onshore ekman pumping through the bbl resulting from increased bottom stress as the southerly flow accelerates when the eac encroaches across the continental shelf

direct ion microprobe u pb dating of fossil tooth of a permian shark abstract we report here direct ion microprobe dating of fossil tooth dentine of a permian fresh water shark orthacanthus senckenbergianus using the shrimp instrument recently installed at hiroshima university fifteen spots on the small sample approximately 2 mm 1 mm indicate a 238 u 206 pb isochron age of 266 18 ma and a tera wasserburg concordia intercept age of 266 24 ma in a three dimensional 238 u 206 pb 207 pb 206 pb 204 pb 206 pb diagram these permian ages are consistent with a 235 u 207 pb age of 453 170 ma and a 232 th 208 pb age of 235 310 ma suggesting indistinguishable depositional and early diagenetic ages of the fossil in its sedimentary sequences the success of the method depends on the chemical fractionation of uranium from lead in a specimen a few hundred microns in size and the consequent variations in lead isotopic compositions due to radioactive decay

high closure temperatures of the u pb system in large apatites from the tin mountain pegmatite black hills south dakota usa abstract the cores of several large 1 2 cm apatites from the early proterozoic tin mountain pegmatite black hills south dakota have concordant u pb ages the age uncertainties of these samples are less than 5 ma the lead isotopic compositions of muscovite and k feldspar coexisting with the apatites were used for initial lead isotopic corrections the resulting u pb concordia ages agree well with u pb and pb pb isochron ages based solely on multiple fragments from single apatites this agreement justifies the use of coexisting muscovite and k feldspar for initial lead corrections the precision of these u pb ages is comparable to those of the u pb systems of more commonly analyzed minerals such as zircon monazite and sphene the estimated u pb closure temperature t c of these apatites is about 620 c this t c is in close agreement with the t c calculated for 0 5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite apatite can therefore yield high precision u pb ages that are close to its crystallization age in igneous rocks

nd o and sr isotopic constraints on the origin of precambrian rocks southern black hills south dakota abstract the nd o and sr isotopic characteristics of precambrian metasedimentary metavolcanic and granitic rocks from the black hills of south dakota are examined two late archean granites 2 5 2 6 ga have t dm ages of 3 05 and 3 30 ga suggesting that at least one of the granites was derived through the melting of significantly older crust early proterozoic metasedimentary rocks have t dm ages that range from 2 32 to 2 45 ga these model ages in conjunction with probable stratigraphic ages ranging from 1 9 to 2 2 ga indicate that mantle derived material was added to the continental crust of this region during the early proterozoic previous studies of the harney peak granite complex have reported u pb and rb sr ages of about 1 71 ga and most granite samples examined in this study have sr isotopic compositions consistent with that age two granite samples taken from the same sill however give two point rb sr and sm nd ages of 2 08 0 08 and 2 20 0 20ga 2200 nd 15 5 respectively in addition whole rock and apatite samples of the spatially associated tin mountain pegmatite give a sm nd isochron age of 2000 100 ma 2200 nd 5 8 1 8 the sm nd o and rb sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post crystallization processes and the age of the pegmatite and parts of the harney peak granite complex remain uncertain processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material assimilation mixing of ree between granite and country rock during crystallization via a fluid phase and post crystallization mobility of sr the nd isotopic compositions of the pegmatite and the harney peak granite indicate that they were not derived primarily from the exposed metasedimentary rocks

petrogenesis of early proterozoic pelitic schists of the southern black hills south dakota constraints on regional low pressure metamorphism the late archean to early proterozoic terrain exposed in the black hills south dakota records a history of deformation metamorphism and igneous activity extending from 2 5 to 1 7 ga metamorphism began during crustal thickening however maximum temperatures were attained after crustal thickening during a regional low pressure high temperature event coinciding with the emplacement of the harney peak granite at 1 7 ga metamorphic grade increased over a distance of 10 to 15 km from garnet grade in the northern part of the terrain to sillimanite k feldspar grade in the southern part isograds mapped in the terrain include the first appearance of staurolite biotite andalusite biotite sillimanite sillimanite biotite garnet and sillimanite k feldspar in the extreme western exposure of the terrain kyanite biotite occurs instead of andalusite biotite geobarometry indicates that metamorphism occurred between 2 0 and 4 4 kbar geothermometry indicates that temperature increased from 469 500 c in the garnet zone to 528 555 c in the sillimanite zone on the northern flank of the harney peak granite higher temperatures are recorded along the southern flank of the granite in the sillimanite biotite garnet zone 585 c and in the sillimanite k feldspar zone 662 c the distributions of isograds and isotherms indicate that the harney peak granite was at the locus of a regional thermal high and that doming during granite emplacement was partly responsible for exposure of the low pressure high temperature metamorphic terrain

age of the dahongliutan rare metal pegmatite deposit west kunlun xinjiang nw china constraints from la icp ms u pb dating of columbite fe and cassiterite abstract dahongliutan is a medium size li be ta nb rare metal pegmatite deposit in the tianshuihai terrane west kunlun orogenic belt nw china in this paper la icp ms u pb ages of columbite fe and cassiterite are reported to constrain the metallogenic age columbite fe with high u and low th yielded a late triassic weighted mean 206 pb 238 u age of 211 9 2 4 ma whilst cassiterite yielded a 206 pb 238 u 207 pb 235 u concordia lower intercept age of 218 12 ma and a tera wasserburg lower intercept age of 218 12 ma which is identical to the columbite fe u pb age and thus represents the emplacement age of the pegmatite dikes the ore hosting pegmatite dikes are intimately time space related to the dahongliutan s type granite ca 220 217 ma indicating that they may have been cogenetic integrating new and published geological data we suggest that the dahongliutan pegmatites may have evolved from the granitic magma represented by the dahongliutan s type pluton in a post collisional tectonic setting regionally the tianshuihai terrane is the western extension of the songpan ganzi block and thus the kunlun li mineralization belt may have been connected to the songpan ganzi li mineralization belt which contains eleven lithium deposits with about 9 8 of li 2 o reserves in china implying that the dahongliutan pegmatite deposit may also have favorable ore forming conditions the similar columbite fe and cassiterite ages demonstrate that these minerals can be precisely dated and could provide useful alternatives to accurately constrain the timing and evolution of rare metal mineralization

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

phosphate ages in apollo 14 breccias resolving multiple impact events with high precision u pb sims analyses abstract the u pb systems of apatite and merrillite grains within four separate apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry no systematic difference was identified between the 207 pb 206 pb ages of the apatites and merrillites a combined 207 pb 206 pb age of 3927 2 ma 95 conf is determined for three of these samples 14305 103 3926 4 ma 14306 150 3926 6 ma 14314 13 3929 4 ma by combining these data with the ages previously obtained for zircons in apollo 12 impact melt breccia fragments and the lunar meteorite sau 169 a weighted average age of 3926 2 ma 95 conf is obtained which is attributed to the formation of the imbrium basin an age of 3943 5 ma is determined for the fourth breccia 14321 134 which is similar to ages of 3946 15 ma and 3958 19 ma obtained from several older phosphates in 14305 103 and 14314 13 the weighted average of these three older ages is 3944 4 ma 95 conf this is indistinguishable to the age 3938 4 ma 2 obtained for a different apollo 14 impact melt breccia in a previous study after investigating likely sources for this older 3940 ma age we conclude that the humorum or serenitatis basin forming events are likely candidates the potential identification of two large impact events within 15 myrs has important implications for the rate of lunar bombardment around 3 95 3 92 ga this study demonstrates the importance of high precision age determinations for interpreting the impact record of the moon as documented in lunar samples

high resolution u pb ages of ca phosphates in apollo 14 breccias implications for the age of the imbrium impact abstract previous age estimates of the imbrium impact range from 3770 to 3920 ma with the latter being the most commonly accepted age of this basin forming event the occurrence of ca phosphates i

variation in sea temperature and the east australian current in the solitary islands region between 2001 2008 abstract improved understanding of the east australian current eac and sea temperature patterns within the solitary islands region of northern new south wales an area where tropical and temperate faunas overlap is an essential step in explaining cross shelf gradients in biotic patterns sea temperature at 10 m was logged using thermistors at seven stations every 30 minutes between january 2001 and december 2008 stations were replicated in three distance from shore categories 6 km from the coast corresponding with predominant assemblage patterns of reef fish daily monthly seasonal and yearly sea temperature patterns were compared between and within stations and distance from shore categories sst images were examined to determine the role of the eac in producing short period 2 to 4 days temperature anomalies sea temperatures ranged between 16 6 27 5 c and were highest offshore and lowest inshore offshore sites experienced average temperatures 1 c higher than nearshore sites over the 8 year study there was considerable variation in sea temperature between years with 2002 and 2006 being the warmest and 2007 the coolest these patterns correspond with strong inter annual variability of the eac at the scale of the solitary islands region the eac influenced shelf waters most strongly during late spring summer when temperatures were also most variable over the smallest temporal scales hours days short period anomalies between and within stations could largely be explained by variable encroachment of the eac across the shelf and or colder intrusions of water forming adjacent to the coastline previous assumptions that the eac strongly influences gradients in the distribution of tropical species in this nearshore region are strongly supported

temperate ice in the shear margins of the antarctic ice sheet controlling processes and preliminary locations abstract the flux of grounded ice from the antarctic ice sheet ais primarily occurs through narrow fast flowing outlet glaciers and ice streams shearing generates heat in the lateral glacier margins where there is a transition from fast flow to near stagnant ice or rock this heat warms the ice and can form zones of water saturated porous ice at the melting temperature known as temperate ice here we derive a one dimensional thermomechanical model to infer which ais shear margins may contain temperate ice our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone both of these quantities are described by two nondimensional numbers the brinkman and peclet numbers that are functions of observable parameters such as shear strain rate surface temperature surface accumulation rate and ice thickness the development of temperate zones is incited by shear strain rates brinkman number and suppressed by the advection of cold ice from the surface peclet number which scales with surface accumulation rate we also include a parametrization for lateral advection which can suppress the formation of temperate ice applying recent observations and outputs from a regional climate model we show that many active glacier shear margins could contain temperate ice the spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates

subinertial currents on the southern califoinia shelf abstract subinertial currents on the southern california shelf are investigated using observations from a current meter array deployed near san diego during the summer fall and winter of 1978 79 this region is characterized by weak winds order 1 2 m s 1 and thus other driving mechanisms for the subinertial currents may be important a simplified depth averaged longshore momentum equation consisting of a balance between current accelerations the adjusted sea level asl gradient and the wind and bottom stress is examined the asl gradient is estimated using sea level and atmospheric pressure observations separated by 350 km a linear parameterization of the bottom stress is used with a drag coefficient of 5 10 4 m s 1 during fall and winter the first order balance of terms in the longshore momentum equation over the inner shelf 15 m depth is between the wind and bottom stress over the outer shelf 60 m the primary balance is between the asl gradient and the bottom stress at midshelf 30 m bo

climatic variability in the southwest pacific during the last termination 20 10 kyr bp the degree to which palaeoclimatic changes in the southern hemisphere co varied with events in the high latitude northern hemisphere during the last termination is a contentious issue with conflicting evidence for the degree of teleconnection between different regions of the southern hemisphere the available hypotheses are difficult to test robustly however because there are few detailed palaeoclimatic records in the southern hemisphere here we present climatic reconstructions from the southwestern pacific a key region in the southern hemisphere because of the potentially important role it plays in global climate change the reconstructions for the period 20 10 kyr bp were obtained from five sites along a transect from southern new zealand through australia to indonesia supported by 125 calibrated c 14 ages two periods of significant climatic change can be identified across the region at around 17 and 14 2 cal kyr bp most probably associated with the onset of warming in the west pacific warm pool and the collapse of antarctic ice during meltwater pulse 1a respectively the severe geochronological constraints that inherently afflict age models based on radiocarbon dating and the lack of quantified climatic parameters make more detailed interpretations problematic however there is an urgent need to address the geochronological limitations and to develop more precise and quantified estimates of the pronounced climate variations that clearly affected this region during the last termination c 2005 elsevier ltd all rights reserved

quantitative mapping of the east australian current encroachment using time series himawari 8 sea surface temperature data the east australian current eac is a highly dynamic western boundary current that exerts significant influences on the marine ecosystem off south east margin of australia for the first time a quantitative mapping of the eac system was conducted using himawari 8 sea surface temperature data and a topographic position index tpi based image processing technique the validation using bluelink reanalysis data suggested good reliability of our mapping results subsequent quantitative examination provides new insights into the eac s cross shelf movement which is a driving force of adjacent slope shelf circulations along the coast of new south wales nsw large scale and high frequency eac encroachment was observed being every 60 80 days upstream 30 32 s and every 90 100 days downstream 33 35 s which is associated with the eac s intrinsic oscillation and eddy shedding downstream the eac encroachment exhibits a 20 day longer period and a double amplitude such dephasing is most likely due to the abrupt change of regime at the eac separation point 32 33 s upstream the eac encroachment also exhibits seasonality being 10 15 km closer to the coast in austral summer which is due to the seasonal eac broadening higher frequency 16 32 days and smaller magnitude eac intrusion was observed along the entire nsw coast 28 37 s which is possibly associated with eac s meanders and frontal eddies in the extension zone 37 30 44 s we observed maximum eac encroachment in summer which is an expression of the seasonal boundary flow off eastern tasmania a dataset of eac encroachment 2015 2017 is available see data availability

the fraser gyre a cyclonic eddy off the coast of eastern australia this paper examines the on shelf circulation of the eastern australian continental shelf for a region off southeast queensland we identify a characteristic seasonally reoccurring wind driven cyclonic flow it influences the cross shelf exchange with the east australian current eac which is the western boundary current of the south pacific ocean we refer to this cyclonic circulation as the fraser gyre it is located south of fraser island between about 25 s and 27 s the region is adjacent to the intensification zone of the eac where the current accelerates and establishes a swift albeit seasonally variable southward boundary flow through the analysis of several data sets including remotely sensed sea surface temperature and sea surface height anomaly satellite tracked surface drifters ocean and atmospheric reanalysis data as well as geostrophic currents from altimetry we find that the on shelf fraser gyre develops during the southern hemisphere autumn and winter months the gyre is associated with a longshore near coast northward flow maximum northward on shelf depth averaged velocities are estimated with about 0 15 0 26 ms 1 the flow turns eastward just to the south of fraser island and joins the persistent southward eac flow along the shelf break the annual mean net cross shelf outward and inward flow associated with the gyre is about 1 17 0 23 sv in the north and 0 23 0 13 sv 1 sv 106 m3s 1 in the south mean seasonal water renewal time scales of the continental shelf are longest during austral winter with an average of about 3 3 days due to the fraser gyre retaining water over the shelf however monthly estimates range from 2 to 8 days with the longer timescale during the austral autumn and winter the southerly wind during austral autumn and winter is identified as controlling the on shelf circulation and is the principal driver of the seasonally appearing fraser gyre the conceptual model of the fraser gyre is consistent with general physical principals of the coastal shelf circulation a southerly wind is associated with surface layer flow toward the coast a near coast positive ssha with a current in the direction of the wind down welling and export of shelf water the fraser gyre influenced cross shelf exchanges are possibly facilitating the offshore transport of fish larvae sediments nutrients river discharges and other properties across the shelf break and into the southward flowing eac during the austral autumn and winter

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

eddy shedding and energy conversions in the east australian current 1 mesoscale variability and eddy shedding in the tasman sea particularly of the east australian current eac is studied through the analysis of remotely sensed observations and outputs from a global ocean model previous observations of the western boundary current separation from the coast showed strong variability at periods ranging between 90 and 140 days we show from satellite altimetric observations that rapid northward migration of the separation point of the eac follows the formation of large eddies at periods of 100 days after an eddy separation event the normally southward flowing current swiftly assumes a more zonal configuration near the separation latitude with a cyclonic circulation developing inshore the formation of large separation eddies is preceded by the southward propagation of sea level anomalies along the east australian continental slope from 25 s sea level anomalies grow as they travel south eventually being pinched off in the form of large anticyclones at 32 s in the current retroflection area energy conversion terms in a global ocean model and in altimetric data suggest both barotropic and baroclinic instability may account for the growth of these anomalies as they propagate south east of the main eac jet there is evidence that eddies may be feeding potential energy back to the mean flow

baroclinic response of sydney shelf waters to local wind and deep ocean forcing a study of the forcing processes responsible for upwelling events in the coastal ocean of sydney australia has been performed using data collected over the summer of 1994 from a shore normal aligned mooring array and a numerical model analyses of the data show that vertical displacements of fluid in the nearshore zone responded principally to the local wind stress during the experimental period however intrusions of mesoscale east australian current features are shown to significantly influence the vertical structure of the water column over the middle and outer shelf regions numerical simulations are performed to investigate the internal processes occurring during strong steady intrusion events of the east australian current onto the sydney shelf these simulations suggest that ekman transport in the bottom boundary layer underlying an intrusion event is not an efficient mechanism for advecting colder water into the nearshore zone on the sydney shelf preconditioning of shelf waters by the east australian current and concurrent forcing by local winds is suggested as an efficient process by which upwelling states may be achieved in the sydney coastal ocean

three dimensional structure of a swarm of the salp thalia democratica within a cold core eddy off southeast australia 1 swarms of the salp thalia democratica periodically occur off southeast australia following the austral spring bloom of phytoplankton in october 2008 a filament of upwelled water was advected south by the adjacent east australian current and formed a 30 km diameter cold core eddy cce the three dimensional structure of a subsurface swarm of t democratica within the eddy was examined using both oblique and vertical hauls and an optical plankton counter opc deployed on a towed body the cce displayed distinct uplift of the nutricline and elevated fluorescence net samples show the zooplankton community was dominated by t democratica comprising 73 88 of zooplankton abundance the size distribution of t democratica measured from net samples was 0 5 5 mm and was used to interpret the opc transects which showed the swarm formed a 15 km diameter disc located 20 40 m deep in the center of the eddy the maximum salp abundance was in the pycnocline and coincided with the subsurface fluorescence maximum the mean abundance of t democratica size particles within the disc was 5003 individuals m 3 ind m 3 contrasted with only 604 ind m 3 at the outer edge of the eddy the vertically concentrated and horizontally constrained disc shaped salp swarm occurred at the interface of salp bearing inner shelf water and nutrient rich upwelled water in a cce the physical processes that formed the cce on the inshore edge of the western boundary current led to the largest density of salps recorded

tectonothermal history of the schwarzwald ore district germany an apatite triple dating approach abstract exceptionally large u rich apatite from the schwarzwald ore district germany was triple dated using id tims u pb fission track ft and u th he dating methods the aim of adopting the multi dating approach on a single sample was to i further constrain thermo tectonic evolution of the schwarzwald from the formation of crystalline basement through to cooling and exhumation and ii test potential and limitations of in situ ft u th he double dating the apatite u pb age of 289 5 4 5 ma records the cooling of the basement through the 550 c during the early permian and brackets the age of the apatite hosting rhyolite between 289 5 4 5 ma and 314 6 ma which is the age of rhyolite hosting granite this is in excellent agreement with radiometric ages of related rhyolites and fits with the magmatic evolution of the schwarzwald during variscan orogeny in the eocene between 50 and 40 ma the basement resided at 100 c and then cooled to surface temperatures as revealed by the ft age 37 6 1 3 ma and thermal modelling results based on track lengths during the cooling event the basement was likely exhumed as a rift shoulder of newly formed upper rhine graben rift nine apatite grains yielded u th he ages from 30 7 1 5 to 65 0 2 6 ma mean age 45 7 1 3 ma three possible reasons for the lack of reproducibility are i non homogeneous distribution of parent isotopes evidenced by strong u zoning ii bad neighbourhood causing implantation of extraneous he and iii radiation damage leading to increased he retentivity implied by the high 250 ppm u concentration these problems could not be circumvented by mechanical abrasion of outermost rim of dated crystals which yielded u th he ages from 26 0 1 0 to 56 2 2 2 ma mean age 37 2 1 4 ma in situ ft and traditional u th he double dating of single apatite crystals revealed that the vast majority of u th he ages were younger than indistinguishable from or overlap within error with corresponding ft ages proving the robustness of this approach the results also show that apparent discrepancies between ft and u th he ages reported in the literature may arise simply as a result of statistical misconceptions and conventional data treatment considering the range of single grain ft ages and the oldest single grain ft age in addition to the central ft age may provide an effective basis on which to evaluate the viability of u th he data

observations of divergence and upwelling around point loma california 1 historical records of near surface water temperatures in the southern californian bight often show a preferential cooling in the lee of headlands such as point dume palos verdes and point loma at times this cooler water is associated with an increase in chlorophyll a as is evident in satellite images of ocean color from the region here we combine hydrographic data from a 1 day cruise aboard the rv roger revelle a precursor to the 0304 california cooperative oceanic fisheries investigations calcofi cruise with high frequency hf radar codar measurements satellite images and long term thermistor records of near surface temperature to identify a small scale isolated upwelling in the lee of point loma 32 5 n associated with the more saline water downstream of the headland are higher nutrient concentrations an increase in chlorophyll a concentration and a bloom of chain forming diatoms indicative of a mature upwelling system it is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the point loma headland time series of surface vorticity calculated from hf radar measurements of sea surface velocity show that as the flow separates from the headland relative vorticity increases offshore of the cape inshore the time series of divergence convergence shows a tendency toward divergence at the surface indicating a preferential upwelling which appears to raise the thermocline thus resulting in a flux of cold nutrient rich water to the surface in the presence of high nutrients and light photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

crystallization conditions and evolution of magmatic fluids in the harney peak granite and associated pegmatites black hills south dakota evidence from fluid inclusions abstract a microthermometric study of inclusions in granites and pegmatites in the proterozoic harney peak granite system identified four types of inclusions type 1 inclusions are mixtures of co2 and h2o and have low salinities on average 3 5 wt nacleq type 2 inclusions are aqueous solutions of variable salinities from 0 to 40 wt nacleq type 3 inclusions are carbonic dominated by co2 with no detectable water and type 4 inclusions consist of 20 to 100 solids with the remaining volume occupied by a co2 h2o fluid many inclusions have a secondary character however a primary character can be unambiguously established in several occurrences of the type 1 inclusions these inclusions were trapped above the solidus and represent the exsolved magmatic fluid the secondary populations of types 1 2 and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions type 4 inclusions are primary and are interpreted to be trapped fluid bearing complex silicate melts that subsequently solidified or underwent other posttrapping changes it is demonstrated that primary type 1 fluid inclusions that coexist with crystallized melt inclusions in the complex li bearing tin mountain pegmatite were trapped along the two fluid phase boundary in the system co2 h2o nacleq consequently the temperature and pressure conditions of trapping are identical to the bulk homogenization conditions on average 340 c and 2 7 kbar these conditions indicate that this li cs rb p and b rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust an internally consistent empirical solvus surface in p t xco2 coordinates was generated for the pseudobinary co2 h2o 4 3 wt nacleq pegmatite fluid system distribution coefficients for the major species co2 h2o nacl and ch4 between the immiscible co2 rich and h2o rich fluid phases as a function of pressure and temperature were extracted from data for the two cogenetic fluid inclusions types

a multielement geochronologic study of the great dyke zimbabwe significance of the robust and reset ages abstract new sm nd u pb and pb pb age determinations indicate that the great dyke of zimbabwe an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide is over 100 ma older than previously believed based on rb sr ages the intrusion was emplaced as a series of subchambers with similar stratigraphy comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite the top sections of which include pt enriched sulfide zones and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite the sm nd method has yielded a combined mineral whole rock isochron of 2586 16 ma and e nd t of 1 1 for samples from the darwendale sebakwe and wedza subchambers as well as the satellite east dyke this isochron age is in excellent agreement with the u pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the selukwe subchamber with an error weighted mean at 2587 8 ma two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant and although not well constrained suggest pb loss from the zircons at ca 830 ma this may be related to the onset of the widespread and diachronous pan african tectonothermal event in southern africa whole rock samples and clinopyroxene and plagioclase separates from a darwendale subchamber drill core yielded a 207 pb 204 pb vs 206 pb 204 pb isochron age of 2596 14 ma which is in agreement with the sm nd isochron and the rutile u pb crystallization age this new age information shows that emplacement of the great dyke and its satellite dikes closely followed the amalgamation of the kaapvaal and zimbabwe cratons and was contemporaneous with emplacement of the youngest of the trondhjemite tonalite granodiorite granitoid suite in the zimbabwe craton assuming that amalgamation of the kaapvaal and zimbabwe cratons was largely by nnw directed convergence it follows that the source of the great dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes isochrons of 206 pb 204 pb vs 238 u 204 pb and 207 pb 204 pb vs 235 u 204 pb yield ages with large errors but well constrained initial pb ratios 206 pb 204 pb 14 15 0 30 and 207 pb 204 pb 15 04 0 06 assuming a two stage model for common lead evolution this result yields a value of 9 5 along with the calculated initial sr and nd isotopic compositions these data are consistent with derivation of the great dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction while a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the great dyke the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source

agulhas current meanders facilitate shelf slope exchange on the eastern agulhas bank large solitary meanders are arguably the dominant mode of variability in the agulhas current observational studies have shown that these large meanders are associated with strong upwelling velocities and affect the shelf circulation for over 100 days per year here 10 year time series from two ocean general circulation models are used to create a composite picture of the agulhas current and its interactions with the shelf circulation in meandering and nonmeandering modes both models show good agreement with the size propagation speed and frequency of observed meanders these composite meanders are then used to examine the response of shelf waters to the onset of large meanders with the use of model output enabling the dynamics at depth to be explored results show a composite mean warming of up to 3 c of depth averaged temperature along the shelf edge associated with an intrusion of the current jet onto the shelf driven by an intensification of the flow along the leading edge of large meanders however this intensification of flow results in cooling of bottom waters driving cold events at the shelf break of 10 c at 100 m thus the intensification of the current jet associated with large meander events appears to drive strong up and downwelling events across the inshore front of the agulhas current facilitating shelf slope exchange

effect of wind on continental shelf carbon fluxes off southeast australia a numerical model 1 a coupled physical biological chemical model is used to study the effect of upwelling favorable and downwelling favorable winds on carbon biogeochemistry on the continental shelf off the southeast australian mainland along the continental shelf from 30 s to 34 s upwelling favorable winds with the aid of bottom ekman transport bring dissolved inorganic carbon dic rich slope waters onto the shelf increasing the carbon held in shelf waters for downwelling favorable winds bottom ekman transport still lifts slope waters onto the shelf but the slope water transport and therefore carbon held is reduced compared with the upwelling scenario under upwelling favorable winds filaments of dic and dissolved inorganic nitrogen din rich water reaching the surface produce an outgassing near the site of upwelling and absorption downstream due to primary productivity in a region of the ocean that is generally absorbing the net effect of upwelling is a reduced absorption of atmospheric co2 as a result of the ratio of deep dic and din 12 2 1 mol c mol n being greater than the redfield ratio 6 625 carbon fluxes in the waters off the southeast australian mainland are variable in space with the transport of continental shelf waters to deep waters occurring mainly where alongshore currents separate from the coast and flow over the 200 m isobath

influence of a western boundary current on shelf dynamics and upwelling from repeat glider deployments the separation zone of a dynamically important western boundary current wbc is resolved through a series of sustained glider deployments along the coastal edge of the jet the comprehensive data set from 23 missions 2008 2014 provides a new high resolution hydrographic climatology which is exploited to understand the spatial extent of dense water uplift and the depth averaged momentum balances across the east australian current eac separation zone the predominantly geostrophic shelf circulation and temperature fields are least most variable upstream downstream where encroachment separation dominates for the first time we resolve the nonlinear advection terms which are considerable in the along shelf momentum balance near bottom water masses indicate dense water uplift as a result of the eac encroachment and separation the data provide both new insight into and a climatology of separation induced uplift and demonstrate a successful model for repeat glider missions in a dynamic wbc environment

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

observed bottom boundary layer transport and uplift on the continental shelf adjacent to a western boundary current western boundary currents strongly influence the dynamics on the adjacent continental shelf and in particular the cross shelf transport and uplift through the bottom boundary layer four years of moored in situ observations on the narrow southeastern australian shelf in water depths of between 65 and 140 m were used to investigate bottom cross shelf transport both upstream 30 s and downstream 34 s of the separation zone of the east australian current eac bottom transport was estimated and assessed against ekman theory showing consistent results for a number of different formulations of the boundary layer thickness net bottom cross shelf transport was onshore at all locations ekman theory indicates that up to 64 of the transport variability is driven by the along shelf bottom stress onshore transport in the bottom boundary layer was more intense and frequent upstream than downstream occurring 64 of the time at 30 s wind driven surface ekman transport estimates did not balance the bottom cross shelf flow at both locations strong variability was found in bottom water transport at periods of approximately 90 100 days this corresponds with periodicity in eac fluctuations and eddy shedding as evidenced from altimeter observations highlighting the eac as a driver of variability in the continental shelf waters ocean glider and hf radar observations were used to identify the bio physical response to an eac encroachment event resulting in a strong onshore bottom flow the uplift of cold slope water and elevated coastal chlorophyll concentrations

u th he ages of phosphates from st severin ll6 chondrite abstract we obtained single grain u th he ages from 14 merrillite and five chlorapatite aggregates from st severin to constrain its low t thermal history in addition we performed 3 he 4 he stepped heating diffusion experiments for merrillite and chlorapatite crystals from guarena chondrite to better constrain he diffusion properties in chondritic phosphates in general among the 19 phosphate grains from st severin the five oldest merrillites and four oldest chlorapatites yielded weighted mean ages of 4412 75 ma 1 mswd 0 34 and 4152 70 ma mswd 0 48 respectively these weighted mean ages overlap with the peaks of the corresponding probability density plots thus likely represent the most pristine u th he ages of st severin the radiogenic 4 he and proton induced 3 he diffusion experiments on guarena chondrite resulted in two well defined linear trends in arrhenius plot for merrillite radius 59 m and chlorapatite 43 m grains error weighted linear regressions of the 3 he data yielded following diffusion parameters e a 135 8 4 8 kj mol and ln d o a 2 5 83 0 66 ln s 1 for merrillite and e a 109 3 9 7 kj mol and ln d o a 2 8 15 1 93 ln s 1 for chlorapatite assuming the analyzed fragments approximate the diffusion domain log d o was calculated as 1 93 0 29 log cm 2 s and 1 19 0 84 log cm 2 s for merrillite and chlorapatite respectively these results indicate higher closure t for merrillite than chlorapatite assuming these results also apply to st severin and that crystal dimensions define the limiting diffusive length scale the closure temperatures of merrillite and chlorapatite in st severin are estimated to be 94 112 c for cooling rates of 0 3 2 6 c ma and 18 c for 0 3 c ma respectively the new single grain u th he ages and 3 he 4 he stepped heating diffusion results combined with previously reported pb pb and 40 ar 39 ar with updated t fcs pairs data indicate that st severin experienced relatively rapid cooling from 477 c down to 100 c at a cooling rate of 2 6 c ma the identified thermal history is generally consistent with the reported cooling path from pu fission track data these results suggest that st severin experienced relatively slow cooling compared to h chondrites which may have resulted from its parent body size being larger than that of the h chondrites

petrologic and geochemical links between the post collisional proterozoic harney peak leucogranite south dakota usa and its source rocks abstract the proterozoic terrane of the black hills south dakota includes the composite harney peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes available dates indicate that granite generation post dated regional metamorphism and deformation that have been attributed to collision of the wyoming and superior cratons at 1760 ma previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites in this study whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks these were then used to model observed trace element compositions of the granites using published mineral melt distribution coefficients model trace element melt compositions using pelitic and graywacke protoliths yield similar results the models reproduce well the observed depletion of transition metals and ba in the granites relative to metasedimentary protoliths the depletion is due mainly to high proportion of biotite with variable amounts of k feldspar in the model residue sr is also depleted in the granites compared to source rocks but to a lesser relative extent than ba this is because of the low biotite melt distribution coefficient for sr and because high proportion of plagioclase in the residue is compensated by high sr concentrations in protoliths rubidium cs and ta behaved as slightly compatible to incompatible elements and therefore were not strongly fractionated during melting of the considered elements only b appears to have been highly incompatible relative to residue during melting the protoliths had sufficient b to allow tourmaline crystallization in those parts of the harney peak granite in which ti concentration was sufficiently low not to enhance crystallization of biotite the reproducibility of observed trace element concentrations in the harney peak granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites this argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post collisional harney peak and other similar peraluminous granite suites

the generation and crystallization conditions of the proterozoic harney peak leucogranite black hills south dakota usa petrologic and geochemical constraints the mineralogy petrology and geochemistry of the proterozoic harney peak granite black hills south dakota were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare element pegmatites the granite was emplaced at 3 4 kbar as multiple sills and dikes into quartz mica schists at the culmination of a regional high temperature low pressure metamorphic event principally along the periphery of the main pluton and in satellite intrusions the sills segregated into granite pegmatite couplets the major minerals include quartz k feldspar sodic plagioclase and muscovite biotite mg no molar mgo mgo feo 0 32 0 38 is the predominant ferromagnesian mineral in the granite s core whereas at the periphery of the main pluton and in the satellite intrusions tourmaline mg no 0 18 0 48 is the dominant ferromagnesian phase almandine spessartine garnet is also found in the outer intrusions there is virtually a complete overlap in the wide concentration ranges of sio2 cao mgo feo sr zr w of the biotite and tourmaline bearing granite suites with no discernable differentiation trends on harker diagrams precluding the derivation of one suite from the other by differentiation following emplacement this is consistent with the oxygen isotope compositions which are 11 5 0 6 for the biotite granites and 13 2 0 8 for the tourmaline granites suggesting derivation from different sources the concentrations of tio2 and possibly ba are higher and of mno and b are lower in the biotite granites the normative orthoclase albite ratio is extremely variable ranging from 0 26 to 1 65 in the biotite granites to 0 01 1 75 in the tourmaline granites very few sample compositions fall near the high pressure watersaturated haplogranite minima eutectic trend indicating that the granites for the most part are not minimum melts generated under conditions with a text h text 2 text o 1 instead most biotite granites are more potassic than the water saturated minima and eutectics and in analogy with experimentally produced granitic melts they are best explained by melting at 6 kbar a text h text 2 text o 1 and temperatures 800 c such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals nabelek et al 1992 several of the tourmaline granite samples contain virtually no k feldspar and have oxygen isotope equilibration temperatures 716 775 c therefore they must represent high temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water saturated haplogranite minima or during fractionation of intruded melts into granite pegmatite couplets accompanied by volatile aided differentiation of the alkali elements the indicated high temperatures a text h text 2 text o 1 the relatively high tio2 and ba concentrations and the relatively low partial 18 text o values of the biotite granites suggest that they were generated by high extent biotite dehydration melting of an immature archean metasedimentary source the ascent of the hot melts may have triggered low extent muscovite dehydration melting of schists higher in the crust producing the high b low ti melts comprising the periphery of the main pluton and the satellite intrusions alternatively the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse with the low partial 18 text o biotite granites generated at a deeper hotter region and the high partial 18 text o tourmaline granites at a higher cooler region of the crust the low ti and high b concentrations in the high partial 18 text o melts resulted in the crystallization of tourmaline rather than biotite which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions

response of an oceanic bottom boundary layer on a slope to interior flow part i time independent interior flow abstract the behavior of oceanic boundary layers on a sloping bottom in the presence of stratification is investigated by the method of direct numerical simulations the navier stokes equations are decomposed into mean and turbulent components with the mean equations involving a slope burger number s n2sin2 f2 where n is the buoyancy frequency is the bottom slope angle and f is the coriolis parameter the influence of the turbulent fluctuations is parameterized as eddy coefficients of viscosity and diffusivity two regimes are considered the first is for cases where the eddy coefficients are constant in time but variable in space and the interior flow is allowed to adjust to the boundary mixing the numerical results agree with analytic theory for two limiting cases the first case is when the mixing extends into the fluid interior and the second is when the mixing is limited to a slab region the second regime involves cases in which the eddy coefficients are determined from a gradient richard

annealing history of zircons from apollo 14083 and 14303 impact breccias abstract breccia boulders scattered around the 25 ma cone crater near the apollo 14 landing site provide a potential source of material ejected from the underlying fra mauro formation which is int

ages and genesis of w sn and ta nb sn w mineralization associated with the limu granite complex guangxi china abstract the limu granite complex spans three granitic stages forming the largest indosinian ta nb sn w ore field in south china there includes quartz vein w sn type 1 mineralization associated with second stage granite as well as ta nb sn w mineralization associated with third stage granite including quartz feldspar vein type 2 pegmatite dike type 3 and disseminated type 4 mineralization to elucidate the magmatic evolution and genesis of these types of mineralization we obtained muscovite 40ar 39ar ages zircon la icp ms u pb ages and lu hf and nd isotopic trace element and geochemical compositions of the limu granite complex the limu granites are peraluminous a cnk 1 1 with negative nd t values 8 5 to 9 7 and tdm2 ages of 1780 1685 ma suggesting that they were derived mainly from partial melting of paleoproterozoic basement remnants of caledonian zircons 429 4 5 6 ma indicate caledonian granite melting in the source low zr hf pb ages of ca 203 ma and 40ar 39ar ages of ca 208 ma respectively we conclude that type 1 mineralization was formed at ca 214 ma and was hydrothermal in origin whereas ta nb sn w mineralization was formed at ca 208 203 ma and was of magmatic hydrothermal origin mantle derived heat from a long lived magma chamber was essential for the high temperature anatexis that formed ta nb sn w rich melts with the combined effects of crustal thickening during the indosinian orogeny local delamination asthenospheric upwelling and deep faulting facilitating the continuous supply of heat

cross shelf dynamics in a western boundary current regime implications for upwelling abstractthe cross shelf dynamics up and downstream of the separation of the south pacific ocean s western boundary current wbc are studied using two years of high resolution velocity and temperature measurements from mooring arrays the shelf circulation is dominated by the east australian current eac and its eddy field with mean poleward depth integrated magnitudes on the shelf break of 0 35 and 0 15 m s 1 up and downstream of the separation point respectively the high cross shelf variability is analyzed though a momentum budget showing a dominant geostrophic balance at both locations among the secondary midshelf terms the bottom stress influence is higher upstream of the separation point while the wind stress is dominant downstream this study investigates the response of the velocity and temperature cross shelf structure to both wind and eac intrusions despite the deep water up to 140 m the response to a dominant along shelf wind stress forcing is a classic two layer ekman structure du

inherited zircon and titanite upb systems in an archaean syenite from southwestern australia implications for upb stability of titanite abstract inherited zircon and titanite have been identified in a syenite from the archaean of southwestern australia conventional and shrimp analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 5 ma for the syenite in addition shrimp analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca 3250 ma inherited component conventional u pb ages on titanite also fall between ca 3250 ma and ca 2650 ma demonstrating that inherited titanite as well as zircon is present in the syenite the syenite has been affected by regional upper amphibolite facies metamorphism at an estimated temperature of 625 650 c retention of the inherited radiogenic pb in the titanite is evidence that the closure temperature for titanite is greater than 650 c the presence of inherited titanite and zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of crustal rocks by a potassic magma at

constraints on the genesis of gold mineralization at the homestake gold deposit black hills south dakota from rhenium osmium sulfide geochronology the homestake gold deposit located in the black hills south dakota usa is one of the largest known hydrothermal gold deposits globally with total mining production exceeding 40 moz au rhenium osmium geochronology of ore associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern black hills arsenopyrite yields a rhenium osmium re os age of 1 736 8 ma mean squared weighted deviation 1 6 consistent with existing age constraints for gold mineralization whereas re os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age this is taken to indicate that the re os arsenopyrite chronometer is robust to at least 400 c whereas the re os pyrrhotite chronometer is likely disturbed by temperatures of 300 350 c the re os arsenopyrite age and initial os ratio 0 28 0 15 are interpreted to indicate that gold was introduced at ca 1 730 ma coincident with the onset of exhumation of crustal blocks and possibly the earliest intrusive phases of harney peak granite magmatism new in situ u pb monazite analyses from an aplite dike in the east central black hills indicate that granite magmatism was a protracted event persisting until at least ca 1 690 ma

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

accelerated subglacial erosion in response to stick slip motion subglacial stick slip motion speeds erosion by hydrofracturing and in other ways as determined from analysis of the growing body of field data microearthquake monitoring commonly detects subglacial earthquakes likely mostly from stick slip motion of debris laden ice over bedrock source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps we calculate that the resulting expansion of higher pressure water in nearby cracks promotes hydrofracturing with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior this mechanism also helps generate the step like topography favoring block plucking this stick slip glacier erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress ice flow velocity and water supply

on the variability of the east australian current jet structure meandering and influence on shelf circulation given the importance of western boundary currents over a wide range of scales in the ocean it is crucial that we understand their dynamics to accurately predict future changes for this we need detailed knowledge of their structure and variability here we investigate the jet structure of the east australian current eac using observations from hf radars and moorings deployed at 30 s 31 s meandering core velocity width and eddy kinetic energy eke are quantified from 4 years of hourly 1 5 km resolution surface current maps 2012 2016 to obtain the most detailed representation of the surface eac jet to date the eac flows predominantly over the 1 500 m isobath 50 km offshore but makes large amplitude displacements eastward every 65 100 days the time scale associated with mesoscale eddy shedding at the eac separation smaller amplitude higher frequency meanders occur every 20 45 days using a coordinate frame that follows the jet we show core velocity and eke exhibit seasonality in both magnitude and variance being maximum in summer 1 55 m s 1 mean core velocity minimum in winter 0 8 m s 1 however it is the eddy shedding time scale that dominates jet variability as the eac moves shoreward shelf temperature and along stream velocity vary linearly with jet movement within 35 km of the core the eac is within this range 75 of the time demonstrating its importance to the shelf circulation temperature and velocity fluctuations at the 70 m 100 m isobath are more influenced by wind eac encroachment with the strongest response occurring when wind and eac act constructively

slope control in western boundary currents an analytic solution is presented for the steady state depth averaged western boundary current flowing over the continental slope by combining three highly idealized models the stommel model the munk model and the arrested topographic wave model the main vorticity balance over the slope is between planetary vorticity advection and the slope induced bottom stress torque which is proportional to ry h21 x where r is the rayleigh friction coefficient h is the water depth and y is the meridional velocity this slope induced torque provides the necessary source of vorticity for poleward flow over the slope its simple interpretation being that vorticity is produced because the bottom stress has to act over the seaward deepening water column the character of the solution depends on the slope a as well as on the assumed bottom drag coefficient and the length scale of the boundary current is it is further shown that if the depth averaged velocity flows along isobaths 2r ba then the stretching of water columns associated with cross isobath geostrophic flow which compensates bottom ekman transport is identical to the slope induced torque by the geostrophic velocities

temperate shelf water dispersal by australian boundary currents implications for population connectivity boundary currents have been recognized as potential drivers of spatial heterogeneity in the ocean because of their role in physical transport and influence on large scale coastal processes in this study we used particle tracking methods in a data assimilating eddy resolving ocean circulation model to determine the effect of multiple boundary currents on connectivity around temperate australia during the austral winter results demonstrated that oceanographic connectivity was asymmetric around australia having greater eastward trajectories due to more favorable ocean boundary currents during this season we validated connectivity patterns with genetic data from an ecologically important species the kelp ecklonia radiata which has greater genetic similarity between the west and south coasts of australia compared with the east coast likely due to predominantly eastward propagule dispersal boundary current circulation was a coarse predictor of kelp genetic connectivity on multigeneration time scales and the nature of these relationships varied among the three boundary current systems according to mean current strength

isotopic peculiarities of an archaean pegmatite union mine mica south africa abstract an isotopic investigation rb sr u pb pb pb and sm nd of the archaean union mine pegmatite northern province south africa is reported the intrusion of this pegmatite devoid of zircon and monazite has been dated at 2912 2 6 ma by pb pb step leaching of magmatic garnet this age is supported by an identical two garnet apatite sm nd isochron of 2917 27 ma a rb sr k feldspar garnet albite isochron of 2023 11 ma corresponds to a later hydro thermal overprint isotope resetting during this overprint was incomplete and revealed that various parent daughter isotope pairs within minerals of the union mine pegmatite behaved in exceedingly different manners under the same physico chemical conditions sr isotope resetting was incomplete and resulted in open system behaviour of the pegmatite k feldspar almost quantitatively lost its radiogenic sr which had accumulated between the time of intrusion and the overprint sr loss from muscovite was variable and incomplete as indicated by a wide range in ages from 2088 74 ma to 2744 13 ma only insignificant portions of this sr were incorporated into adjacent albite as revealed by a micro drilled rb sr profile through a muscovite albite grain boundary the 87 sr 86 sr initial ratio is 0 8301 11 for the proterozoic isochron which is far less radiogenic than the calculated value of 2 24 for a closed system it is therefore concluded that sr was lost from the system and that the rb sr ratio of the whole rock was changed apatite on the other hand seems to have behaved as a closed system and retains a low initial sr ratio of 0 7125 u pb analyses of apatite reveal that this mineral has behaved as an almost closed system for pb diffusion as well but was open for u significant loss during the overprint this has resulted in high reverse discordance of apatite u pb analysis and in a geologically meaningless secondary pb pb isochron the data demonstrate that both preferential loss of daughter sr over parent rb and parent u over daughter pb isotopes can occur in some minerals of one and the same rock this process altered the parent daughter ratios to such an extent that in the case of apatite a secondary pb pb isochron with an erroneous age of 4119 66 ma was established it appears likely that preferential loss of daughter or parent isotopes is the reason for spurious secondary whole rock isochrons reported and reviewed by moorbath and taylor 1986 and moorbath et al 1986 such losses far exceeding predictions based on volume diffusion experiments may occur by means of fast transport diffusion which in part may depend on the micro textures of the host minerals geochemical studies of early archaean rocks require substantial back correction for radiogenic decay and the example of the union mine pegmatite shows that changes of parent daughter isotope ratios may occur long after rock formation however such changes may go unnoticed in high precision geochronology i e garnet dating in this study but will lead to erroneous correction for radiogenic decay and hence to erroneous conclusions regarding the origin of the studied samples

a conceptual surrogacy framework to evaluate the habitat potential of submarine canyons the seascape of the vast australian continental margin is characterised by numerous submarine canyons that represent an equally broad range of geomorphic and oceanographic heterogeneity theoretically this heterogeneity translates into habitats that may vary widely in their ecological characteristics here we describe the methodology to develop a framework to broadly derive estimates of habitat potential for pelagic and epibenthic species including demersal fishes and benthic infauna in all of australia s known submarine canyons our analysis shows that the high geomorphic and oceanographic diversity of australian submarine canyons creates a multitude of potential habitat types in general it appears that canyons may be particularly important habitats for benthic species canyons that incise the shelf tend to score higher in habitat potential than those confined to the slope canyons with particularly high habitat potential are located mainly offshore of the great barrier reef and the nsw coast on the eastern margin of tasmania and bass strait and on the southern australian margin many of these canyons have complex bottom topography are likely to have high primary and secondary production and have less intense disturbance to sediment the framework presented here can be applied once refined and comprehensively validated with ecological data to help managers make informed conservation decisions especially for high value canyons

the comparative behavior of apatite zircon u pb systems in apollo 14 breccias implications for the thermal history of the fra mauro formation we report secondary ion mass spectrometry sims u pb analyses of zircon and apatite from four breccia samples from the apollo 14 landing site the zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two sims u pb analyses show that the ages of zircon grains range from 4023 24 ma to 4342 5 ma whereas all apatite grains define an isochron corresponding to an age of 3926 3 ma the disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite u pb systems have been completely reset at 3926 3 ma whereas the u pb system of zircon has not been noticeably disturbed at this time the apatite u pb age is slightly older than the ages determined by other methods on apollo 14 materials highlighting need to reconcile decay constants used for the u pb ar ar and rb sr systems we interpret the apatite age as a time of formation of the fra mauro formation if the interpretation of this formation as an imbrium ejecta is correct apatite also determines the timing of imbrium impact the contrast in the pb loss behavior of apatite and zircon places constraints on the temperature history of the apollo 14 breccias and we estimate from the experimentally determined pb diffusion constants and an approximation of the original depth of the excavated samples in the fra mauro formation that the breccias experienced an initial temperature of about 1300 1100 c but cooled within the first five to ten years

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

upb systematics of stilbite bearing low temperature mineral assemblages from the malmberget iron ore northern sweden minerals with a low thermal stability strongly constrain the history of cooling and later tectonic reworking of an area provided these minerals can be dated the possible use of stilbite a ca al silicate of the zeolite group for geochronologic studies was investigated open fractures in the palaeoproterozoic malmberget iron ore northern sweden contain low temperature mineral assemblages with various combinations of apatite stilbite calcite biotite and less commonly titanite and monazite two generations of fractures that are characterized by calcite and stilbite with distinctly radiogenic initial 87sr 86sr at ca 0 720 and ca 0 708 are dated at ca 1740 ma monazite and 1620 1613 ma titanite respectively apatite samples even those intimately intergrown with ca 1740 ma old monazite yield u pb ages at 1620 1600 ma which indicates that apatite apparently recrystallized and reset its u pb system older stilbite yields a secondary lead isochron at 1730 6 4 ma 2 which unequivocally demonstrates that the ambient temperature in the malmberget area from then on remained below the thermal stability of stilbite ca 150 c stilbite is a natural ion exchanger and its u pb systematics indicates recent mobility of uranium and lead however the 1730 6 4 ma 2 age demonstrates that some of the older stilbite was not disturbed during younger fracturing hydrothermally altered and secondary stilbite samples yield scattered lead arrays that correspond to secondary isochrons at ca 1650 1600 d4a which agrees with the u pb titanite and apatite ages thus in combination with other geochronometers the generally imprecise stilbite ages provide information on the cooling history of an area

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

crustal melts below 400 c we propose that the internally zoned li bearing tin mountain pegmatite in the harney peak granite pegmatite system of the black hills south dakota crystallized from fluid rich compositionally complex melts at 400 350 c the low crystallization temperatures resulted from the combined fluxing effects of li b p h 2 o and carbonate anions the presence of hydrous silicate melts at temperatures of 350 c is revealed by microthermometric data on primary fluid inclusions cogenetic with crystallized melt inclusions mineral assemblages of the crystallized melt inclusions and chemistry of bulk fluid leachates indicate that the melts and fluids contained significant amounts of li and na as carbonates and or borates that acted as powerful fluxes the low temperatures give a new perspective on the stability of silicate melts in the crust

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

beryllium mineral evolution beryllium is a quintessential upper crustal element being enriched in the upper crust by a factor of 30 relative to primitive mantle 2 1 vs 0 07 ppm most of the 112 minerals with be as an essential element are found in granitic pegmatites and alkalic rocks or in hydrothermal deposits associated with volcanic and shallow level plutonic rocks and skarns because of the extensive differentiation needed to enrich rocks sufficiently in beryllium for beryllium minerals to form these minerals are relative latecomers in the geologic record the oldest known is beryl in pegmatites associated with the sinceni pluton swaziland 3000 ma in general beryllium mineral diversity reflects the diversity in the chemical elements available for incorporation in the minerals and increases with the passage of geologic time furthermore the increase is episodic that is steep increases at specific times are separated by longer time intervals with little or no increase in diversity nonetheless a closer examination of the record suggests that at about 1700 ma the rate of increase in diversity decreases and eventually levels off at 35 species formed in a given 50 ma time interval between 1125 and 475 ma then increases to 39 species at 125 ma except for four spikes before dropping off to 30 species for the last 100 ma these features appear to reflect several trends at work 1 diversifications at 2475 1775 and 525 ma which are associated with highly fractionated rare element granitic pegmatites and with skarns at langban and similar deposits in the bergslagen ore region of central sweden and which are inferred to correspond to the collisional phases of the supercontinents kenorland nuna and gondwana respectively 2 diversification at 1175 ma due to the rich assemblage of beryllium minerals in the ilimaussaq peralkaline complex gardar province west greenland in an extensional environment 3 diversification at 275 ma which is largely attributable to granitic pegmatites appalachian mountains u s a and urals russia and the larvik alkalic complex norway but nonetheless related to continental collision and 4 limited exhumation of environments where beryllium minerals could have formed in the last 100 ma that the maximum diversity of be minerals in any one geologic environment could be finite is suggested by the marked slowing of the increase in the number of species formed in a given 50 ma time interval whereas the drop off at 100 ma could be due to 100 myr being too short a time interval to exhume the deep seated occurrences where many be minerals had formed the relative roles of chance vs necessity in complex evolving systems has been a matter of considerable debate one equally applicable to what extent the temporal distribution of beryllium minerals is a matter of contingency on the one hand the appearance of the most abundant be minerals such as beryl and phenakite early in the history of be mineralization appears to be a deterministic aspect since these minerals only require the abundant cations al and si and crystallize at relatively low concentrations of be in aqueous solution or granitic magmas on the other hand it could be argued that the very existence of most other be minerals as well as the temporal sequence of their appearance is a matter of chance since 55 of the 112 approved be minerals are known from a single locality and many of these phases require an unusual combination of relatively rare elements consequently we cannot exclude the possibility that other equally rare and thus contingent potential be minerals await discovery in as yet unexposed subsurface deposits on earth and we suggest that details of be mineral evolution on other earth like planets could differ significantly from those on earth

topographically induced upwelling off eastern australia abstract a high resolution numerical study of an idealized western boundary current flow over variable topography is presented with application to the east australian current eac the results indicate that alongshelf topographic variations off australia s east coast cause the eac to accelerate over the narrowing continental shelf near cape byron this acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer bbl which would usually cause the eac driven bbl to shut down consequently a region of persistent high bottom stress was established off cape byron which was responsible for driving an upwelling bbl it is shown that the enhanced vertical mixing associated with a low richardson number flow beneath the eac reduced the local stratification consequently the burger number is decreased resulting in a long shutdown timescale of the bbl which enables a nearshore thermal to be established and maintained such fronts are commonly observed in the region as a p

trace element constraints on pegmatite genesis tin mountain pegmatite black hills south dakota the tin mountain pegmatite is a small zoned granitic body that is extremely enriched in rb and li but has moderate concentrations of sr and ba these trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less evolved granitic melt to have produced the parental liquid to the tin mountain pegmatite batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in rb and li to be the parental liquid or that had sr and ba concentrations that were much higher than those estimated for the parental liquid the modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated harney peak granite gives calculated melt compositions with either lower sr and ba concentrations or inadequate rb and li enrichments to be the parent liquid of the pegmatite at least two variants from simple crystal liquid fractionation models can however successfully account for the derivation of the parent liquid 1 generation of a rb li ba and sr rich granitic melt outside of the compositional range of the sampled portions of the harney peak granite complex by low degrees of partial melting of metasedimentary rocks found in the black hills followed by moderate extents of fractional or equilibrium crystallization 2 derivation from harney peak granite via a complex multi stage crystal liquid fractionation process such as progressive equilibrium crystallization

seasonality of sporadic physical processes driving temperature and nutrient high frequency variability in the coastal ocean off southeast australia physical processes forced by alongshore winds and currents are known to strongly influence the biogeochemistry of coastal waters combining in situ observations moored platforms hydrographic surveys and satellite data sea surface wind and sea surface height we investigate the transient occurrence of wind driven upwelling downwelling and current driven upwelling events off southeast australia remote sensed indices are developed and calibrated with multiannual time series of in situ temperature and current measurements at two shelf locations based on archives up to 10 years long climatological analyses of these indices reveal various latitudinal regimes with respect to seasonality magnitude duration of events and their driving mechanisms wind or current generally downwelling favorable winds prevail in this region however we demonstrate that up to 10 wind driven upwelling days per month occur during spring summer at 28 33 5 degrees s and up to 5 days in summer further south current driven upwelling upstream of the east australian current separation zone approximate to 32 degrees s occurs twice as often as downstream using independent in situ data sets we show that the response of the coastal ocean is consistent with our climatology of shelf processes upwelling leads to a large range of temperatures and elevated nutrient concentrations on the shelf maximized in the wind driven case while downwelling results in destratified nutrient poor waters the combination of these sporadic wind and current driven processes may drive an important part of the high frequency variability of coastal temperature and nutrient content our results suggest that localized nutrient enrichment events of variable magnitude are favored at specific latitudes and seasons potentially impacting coastal ecosystems key points multisensor analysis of shelf processes combining in situ and satellite data spatio temporal variability of transient wind and current driven up downwelling cold nutrient rich water intrusions favoured at specific locations seasons

evaluation of globcurrent surface ocean current products a case study in australia abstract australia s marginal seas include a wide range of ocean current regimes ranging from tide dominated in the north west where the continental shelf is wide to boundary current and eddy dominated in the south east where the shelf is narrow here we take the opportunity to test the globcurrent surface ocean current products against in situ observations in these two contrasting regimes observations by acoustic doppler current profiler adcp instruments of australia s integrated marine observing system imos and drifting buoys of the global drifting programme gdp are used the monthly timescale variability of the globcurrent alongshore current component is in moderately good agreement with the observations on the continental shelf in the south east but neither the shorter period variability nor the long term mean are representative of the adcp observations while the observed tidal currents are negligibly small the wind driven signals are not but these are evidently too transient to be adequately sampled by altimetry the inclusion of an ekman component does not represent these signals because the interaction of the ekman transport with the coastal boundary condition is not included similarly the error of the time mean velocity which is the dominant error is because that product is not designed to represent the highly anisotropic nature of the sea level gradients over the continental shelf nor the constraints on the flow field that are imposed by the topography we thus conclude that the globcurrent product needs improvements before it can be described as very suitable for applications on this and probably other narrow continental shelf off the continental shelf in contrast the globcurrent products compare quite well with the trajectories of drifting buoys confirming that the products are quite suitable for blue water applications in contrast to the south east the tides are very strong in the north west region of australia the sub tidal variability is weak in both relative and absolute senses consequently the removal of the tidal signal from the sea level observations needs to be very complete for the residual error to be smaller than the true sub tidal signal transient wind forced signals are also occasionally large so this step of the de aliasing also needs to be very accurate unfortunately it appears that more work is required before accurate estimates of sub tidal variability are available from globcurrent the magnitude of the globcurrent estimates of sub tidal current variability far exceed the magnitude of and are uncorrelated with the detided adcp data

classification of submarine canyons of the australian continental margin abstract submarine canyons influence oceanographic processes sediment transport productivity and benthic biodiversity from the continental shelf to the slope and beyond however not all canyons perform the same function the relative influence of an individual canyon on these processes will in part be determined by its form shape and position on the continental margin here we present an analysis of canyon geomorphic metrics using an updated national dataset of 713 submarine canyons surrounding mainland australia these metrics attributes for each canyon are used to classify them into canyon types across a hierarchy of physical characteristics separately for shelf incising n 95 and slope confined blind n 618 canyons we find that the canyon metrics describe a wide variety of canyon form and complexity that is consistent with a population of canyons that has evolved at different rates around the australian margin since the break up of gondwana the large number of slope confined canyons is interpreted to reflect dominance of slope mass wasting processes over erosive turbidity flows from fluvial and shelf sources on an arid continent the distribution of submarine canyons around the australian margin is not regular with clusters occurring in the east southeast west and southwest where the margin is steepest the classification result provides a quantitative framework for describing canyon heterogeneity for application in studies of geological controls on individual canyons canyon oceanography and canyon biodiversity

lead isotopic evidence for mixed sources of proterozoic granites and pegmatites black hills south dakota usa abstract the lead isotopic compositions of k feldspars separated from the ca 1700 ma harney peak granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources it has been reported previously that the core of the harney peak granite complex is dominated by relatively lower gd18o avg 11 5 granites whereas higher gd18o avg 13 2 granites occur around the periphery of the complex the higher 18 o granites and one simple pegmatite have low values of 207 pb 204 pb for their 206 pb 204 pb thus they likely were derived from a source with a short crustal residence time this source may have been the pelitic schists into which the harney peak granite complex and pegmatites were intruded feldspars from granites with lower gd18o values have significantly higher 207 pb 204 pb for their 206 pb 204 pb the data define a linear array with a slope equivalent to an age of ca 2 6 ga with t2 defined to be 1 7 ga such a slope could represent a mixing array or a secondary isochron for the source these low 18o granites could have been derived from a source with a high u pb and with a crustal residence beginning before the proterozoic the source s of these granites may have been a sediment derived from late archean continental crust the highly evolved tin mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups suggesting either a mixed source or contamination two late archean granites the little elk granite and the bear mountain granite had precursors with high u pb and low th u histories the th u history of the bear mountain granite is too low for this rock to have been an important component of the source of the proterozoic granites however crustal rocks with lead isotopic compositions similar to those of the little elk granite were an important source of lead for some of the proterozoic granitic rocks

palaeoproterozoic thermal events recorded in the 4 0 ga acasta gneiss canada evidence from shrimp u pb dating of apatite and zircon u pb isotopes of apatites and zircons extracted from a sample of the acasta gneisses that occurs along the western margin of the slave craton in northwest territories of canada have been measured using a shrimp ii ion microprobe eleven apatite analyses 30 m spot give 238u 204pb 206pb 204pb and 204pb 206pb 207pb 206pb isochron ages of 1905 86 ma 2 and 1936 28 ma 2 respectively analyses 20 m spot on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238u 206pb 207pb 206pb concordia plot with concordia intercepts at 4014 25 ma 2 and 1967 93 ma 2 the former agrees with 3 96 4 02 ga protolith ages obtained on the oldest components of the acasta gneisses by other workers the lower concordia intercept for the zircon data is consistent with the u pb age of apatites and may be related to a 500 c thermal event perhaps early in the palaeoproterozoic wopmay orogeny