circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

geologic and geochemical relationships between the contact sublayer inclusions and the main mass of the sudbury igneous complex a case study of the whistle mine embayment more than half of the giant ni cu platinum group element pge sulfide ores of the sudbury igneous complex are associated with a discontinuous unit at the base of the main mass known as the sublayer the sublayer is comprised of two fragment rich members 1 a metamorphic textured footwall breccia and 2 an igneous textured contact sublayer the contact sublayer occurs as a thick unit in depressions at the base of the sudbury igneous complex termed embayments it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase melanorite olivine melanorite metamorphosed melanorite wehrlite and dunite we present data for a case study of the whistle mine embayment and show that the igneous textured sublayer matrix is geochemically unlike the main mass norites quartz gabbros and granophyres for example the igneous textured sublayer matrix in the whistle mine has la sm 4 0 la nb 5 1 and th zr 0 02 and the main mass norites quartz gabbros and granophyres have ratios of 5 5 to 7 2 8 to 4 2 and 0 04 to 0 05 respectively this matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase the igneous textured sublayer matrix at the whistle mine can be modeled with small amounts of assimilation of local country rock granitoids approximately 10 large degrees of assimilation of diabase that are not derived from the immediate country rocks approximately 70 and small contributions from the main mass magma type approximately 20 mafic norite the amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma even with a superheated magma 70 percent assimilation is too large a model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments these fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer there are significant differences in the composition of the igneous textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments the melanorite inclusions in the sublayer at the whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous textured sublayer matrix but they have similar high incompatible element concentrations e g olivine melanorites have 20 65 ppm ce in rocks with 15 21 wt mgo 1 to 10 percent interstitial sulfide up to 0 5 percent apatite 1 to 15 percent biotite and 1 85 ga age zircon and baddeleyite the mafic ultramafic inclusions are interpreted to be the broken up remnants of an earlier cumulate formed at depth from a main mass magma this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase olivine compositional data for the mafic inclusions fo sub 71 79 500 3 800 ppm ni in the absence of reequilibration indicate that olivine crystallization both predated and postdated sulfur saturation of the magma cr rich spinels from these rocks confirm that the parental magma was especially cr rich and based on the olivine compositional data had a tholeiitic mg fe ratio

isotope data from ice station weddell implications for deep water formation in the weddell sea helium isotope data 3he 4he ratios and 4he concentrations and h218o h216o ratios obtained from stations occupied during the drift of ice station weddell february to june 1992 are used together with hydrographic data to study formation of deep and bottom water in the western weddell sea the data indicate deep and bottom water formation along the entire track of the ice station 71 4 to 65 8 s 53 w ice shelf water isw seems to contribute significantly to the formation of weddell sea deep water wsdw and weddell sea bottom water wsbw in the southern part of the drift track toward the north the fraction of isw contained in wsdw wsbw decreases this trend is overlaid by high isw fractions in the deep and bottom waters found in the vicinity of the larsen ice shelf the fraction of western shelf water wsw in wsbw shows the opposite trend increasing from south to north the combined fraction of isw and wsw in waters with potential temperatures below 0 c is about 20 corresponding to a roughly 200 m thick layer overall wsw seems to contribute approximately 2 to 3 times more water than isw to the water column below the 0 c isotherm using the estimated flow of isw over the sill north of the filchner depression of 1 sv foldvik et al 1985a together with the ratio of wsw plus winter water ww to isw we calculate a value of about 5 sv for the formation rate of wsbw with a potential temperature of 0 7 c about one third of this flux represents near surface waters wsw ww which have recently been equilibrated with the atmosphere whereas pure isw contributes about 10

strain trajectories above the main central thrust himalaya in southern tibet as a result of the collision of india with asia1 3 a thick pile of precambrian crystalline rocks the main central sheet was transported southwards over the indian continent a marked linear fabric a mineral stretching lineation within the main central sheet varies only slightly from a north south trend which is interpreted as paralleling the direction of transport of the nappe3 6 our study sought to extend the previous observations to the hitherto poorly known part of the main central sheet in southern tibet china the stretching lineation is dominantly north south in the lower part of the sheet but exhibits continuous and discontinuous variations from north south to east west in the upper part7 these variations of lineation trajectory indicate two shear components one subparallel to and one at a high angle with the himalayan belt the recognition of westward wrench shearing in the main central sheet implies a component of relative displacement parallel to the himalayan belt and not only a radial one as generally assumed in previous models this suggests that the kinematic story of the intracontinental deformation following the continental collision is probably not one of simple north south convergence

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

chlorofluorocarbon derived formation rates of the deep and bottom waters of the weddell sea the spreading and mixing of deep and bottom waters from the weddell sea originating with potential temperature 0 c are investigated through analysis of new and historical cfc measurements we investigate the component that enters the southern indian ocean with a simple one dimensional advection diffusion model of the deep boundary current this model does not require a condition of mixing solely with tracer free water to be imposed as boundary conditions we derive time histories for the cfc concentrations of newly formed weddell sea deep and bottom waters a range of best fit velocities is obtained by determining which model velocities produce model tracer concentrations closest to measured concentrations from this we derive a formation rate of 3 2 1 5 sv for the waters originating with temperatures lower than 0 c that subsequently enter the southern indian ocean this compares to an upper limit for these waters of 5 7 sv derived from the cfc 11 cfc 12 ratio whilst the former value is dependent on the highly idealized concepts of the model used the latter is a more robust upper limit using the cfc 11 cfc 12 ratio measured at vema channel during the world ocean circulation experiment cruise a23 an upper limit of 0 9 sv is derived for the formation rate of waters colder than 0 c that enter the south atlantic overall an upper limit of 6 6 sv is derived for the rate of formation of the deep and bottom waters of the weddell sea a better representation is obtained by combining the upper limit to the south atlantic component with the model derived southern indian ocean estimate we thus estimate the formation rate of the deep and bottom waters in the weddell sea to be 3 7 1 6 sv whilst the range obtained is large and the assumptions implicit in the calculations are potentially significant we believe this more realistically represents the true formation rate than estimates derived using an imposed condition of tracer free mixing the range can be narrowed in future as more data become available

successive overprinting granulite facies metamorphic events in the anmatjira range central australia the anmatjira range and adjacent reynolds range central australia comprise early proterozoic metasediments and othogneisses that were affected by three and possibly four temporally distinct metamorphic events m1 4 and deformation events d1 4 in the period 1820 1590 ma the north western portion of the range around mt stafford preserves the effects of 1820 ma m1 d1 and shows a spectacular lateral transition from muscovite quartz bearing schists to interlayered andalusite bearing migmatites and two pyroxene granofelses that reflect extremely low pressure granulite facies conditions over a distance of less than 10 km orthopyroxene cordierite garnet k feldspar quartz bearing gneisses occur at the highest grade implying peak conditions of 750 c and 2 5 0 6 kbar an anticlockwise p t path for m1 is inferred from syn to late d1 sillimanite overprinting andalusite petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages the effects of m1 have been variably overprinted to the south east by a c 1760 ma m2 d2 event much of the central anmatjira range around ingellina gap comprises orthogneiss deformed during d2 and metapelites that have m1 andalusite and k feldspar overprinted by m2 sillimanite and muscovite the south eastern portion of the range around mt weldon comprises metasediments and orthogneisses that were completely recrystallized during m2 d2 with metapelitic gneisses characterized by spinel sillimanite k feldspar quartz bearing assemblages that suggest peak m2 conditions of 750 c and 5 5 1 kbar overprinting parageneses in metapelitic gneisses imply that d2 occurred during essentially isobaric cooling a third granulite facies event m3 affected rocks in the reynolds range immediately to the south of the anmatjira range at c 1730 ma a possible fourth event m4 with a minimum age of c 1590 my affected both ranges but resulted in only minor overprinting of m1 3 assemblages the superimposed effects of m1 4 mapped for the entire anmatjira reynolds range area indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding events although the immediate cause of each of the metamorphic events involved advection the ultimate causes were external to the metasediments and most probably external to the crust

shallow impact isotopic insights into crustal contributions to the sudbury impact melt sheet abstract the largest known terrestrial impact melt sheet occurs within the 1850 ma sudbury structure ontario in order to evaluate the relative contributions of different target lithologies to the melt sheet we have investigated the pb isotope compositions of feldspar separates from early formed quartz diorite magmas within offset dykes from around the impact structure the samples define a linear array on plots of age corrected 206 pb 204 pb versus 207 pb 204 pb samples from offset dykes hosted by the huronian supergroup south range have a range of 206 pb 204 pb 1850 from 15 424 to 17 255 and 207 pb 204 pb 1850 from 15 390 to 15 801 whilst those hosted by archean gneisses of the superior province north range cluster around 206 pb 204 pb 1850 14 8 and 206 pb 204 pb 1850 15 1 these values can be approximated by binary mixing between the two major groups of target lithologies a mix of 60 70 of superior province gneisses with 30 40 of huronian metasedimentary material closely matches the pb isotope compositions of north range offset dyke samples whereas in the south range the required huronian component is up to ca 80 these mixing proportions are consistent with sr nd and os isotope and trace element constraints a third minor component either locally exposed paleoproterozoic mafic rocks or the lower crust is also required however the isotopic trace element and ni cu platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component a major contribution of huronian supracrustal material which had a pre impact thickness of up to 12 km is required to explain the chemical characteristics of the impact melts which also have a strong upper crustal affinity e g eu sm 0 22 rb sr 0 2 0 35 as such a shallower level of melting is apparent than that predicted by many previous impact models for the sudbury event this can be accommodated by considering approach trajectories for the impactor oblique to the earth s surface in addition the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage and may not have been completely homogenised during crater formation our findings have significant implications for the nature of the sudbury impact event the evolution of the melt sheet and the crustal sources of metals contained in sudbury s world class ni cu pge sulphide ores

precambrian tectonics and the physicochemical evolution of the continental crust i the gabbro eclogite transition revisited abstract we report the first occurrence of retrograde eclogite shear zones cutting proterozoic granulites from australia the musgrave range granulites were metamorphosed at about 12 kbar pressure and 850 900 c the granulites then cooled isobarically and developed a variety of lower temperature mineral reaction coronas typical of garnet granulites these rocks are cut by a major shear zone the davenport shear in which lower temperature fine grained eclogite developed this demonstrates that mafic rocks in the deep crust can cool down into the thermodynamic stability field of eclogite without undergoing transformation to eclogite we agree with harte et al 1981 that rocks lacking a fluid phase in stable tectonic regions are unlikely to maintain local equilibrium at ambient geotherm conditions until depths well within the upper mantle are reached it is proposed that many isobaric high pressure granulites could have cooled down into the eclogite field before their later excavation to the surface and did not transform to an eclogite mineralogy the inability of the lower crust to transform into eclogite by retrograde reaction has important implications for the seismic petrologic and geochemical character of the lower crust the continental moho cannot be caused by the gabbro eclogite transition preservation of relatively low density high temperature granulite and gabbroic mineral assemblages in the lower continental crust prohibits major recycling of lower crust back into the mantle once it is removed from active plate boundaries however in island arcs with magmatically thickened crust mafic rocks at a depth of 60 to 70 km will be in the eclogite stability field at high enough temperatures 800 900 c for eclogite to rapidly form and sink back into the mantle this could result in a net accreted crust of broadly andesitic composition in contrast later growth of continental crust by basalt underplating in stabilised shield regions will not result in subsequent eclogite formation and lower crustal recycling back into the mantle will not occur thus estimates of the average composition of the continental crust which are based on a mass balance of plate margin basaltic input from the mantle will not be adequate

evolution of the sublayer of the sudbury igneous complex geochemical sm nd isotopic and petrologic evidence abstract the mineralized sublayer at the base of the sudbury igneous complex sic consists of two variants the noritic contact sublayer and radial and concentric quartz dioritic offset dykes both are characterized by the presence of significant quantities of ni cu pge sulphides and by a prominent population of recrystallized diabasic textured and melanocratic to ultramafic fragments the two variants of the sublayer contain compositionally distinct inclusion populations and inclusion bearing matrices contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments the compositional variation in sic rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt s the basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase and is geochemically and isotopically identified with huronian basalts which comprise the south range footwall with ce yb n ratios of around 2 5 and e nd 1850 between 2 to 5 the melanocratic inclusions in the sublayer are typically coarse grained and undeformed with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet which has ce yb n ratios of around 10 and e nd 1850 around 9 calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet it is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact this basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling offset dykes were emplaced and the magmas effectively removed from the system subsequently continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer no extracrustal e g mantle component is envisioned in this model to explain the silicate compositional distributions in the sic and mafic crustal rocks in the target zone are implicated as the metal source for the sic deposits

a seasonal cycle in the export of bottom water from the weddell sea dense water formed over the antarctic continental shelf rapidly descends into the deep ocean where it spreads throughout the global ocean as antarctic bottom water1 2 the coldest and most voluminous component of this water mass is weddell sea bottom water1 3 4 5 6 7 here we present observations over eight years of the temperature and salinity stratification in the lowermost ocean southeast of the south orkney islands marking the export of weddell sea bottom water we observe a pronounced seasonal cycle in bottom temperatures with a cold pulse in may june and a warm one in october november but the timing of these phases varies each year we detect the coldest bottom water in 1999 and 2002 whereas there was no cold phase in 2000 on the basis of current velocities and water mass characteristics we infer that the pulses originate from the southwest weddell sea we propose that the seasonal fluctuations of weddell sea bottom water properties are governed by the seasonal cycle of the winds over the western margin of the weddell sea interannual fluctuations are linked to the variability of the wind driven weddell sea gyre and hence to large scale climate phenomena such as the southern annular mode and el nino southern oscillation

repeated cfc sections at the greenwich meridian in the weddell sea 1 repeated observations of the tracer chlorofluorocarbon 11 cfc 11 for a section along the greenwich meridian from antarctica 70 s to about 50 s are presented for the period 1984 1998 the cfc sections display a highly persistent pattern a middepth cfc minimum in the central weddell sea is bounded laterally by elevated levels of dissolved cfcs at the southern margin of the weddell basin and by a column of elevated cfc concentrations around 55 s near to the northern margin part of the latter column covers waters of the antarctic circumpolar current which indicates that a moderate portion of these waters was ventilated in the weddell sea deep cfc maxima adjoining the southern and northern margins of the weddell basin indicate advective cores of recently ventilated waters the southern core supports previous notions of deep water import into the weddell sea from the east for all deep and bottom waters the portions ventilated on the cfc timescale 50 years are small effective initial cfc saturations for these portions are estimated to be between 60 and 70 using in part new data from off the filchner ronne ice shelf for various cfc features along the section mostly advective cores ventilated fractions and mean ages of these fractions were obtained with error limits the procedure was to fit an age distribution of a prescribed form to cfc 11 time series for these features constructed from the various realizations of the cfc section the ages are between 3 and 19 years and the ventilated fractions range between 6 and 23 indicating a rather limited ventilation of the interior weddell sea subsurface layer waters on the cfc timescale it is shown that the concurrent cfc 12 data provide little additional information the work demonstrates a high information content of repeated tracer observations and encourages similar approaches also in other ocean regions

shock metamorphic features in mafic and ultramafic inclusions in the sudbury igneous complex implications for their origin and impact excavation

calculation of co 2 activities using scapolite equilibria constraints on the presence and composition of a fluid phase during high grade metamorphism thermodynamic and phase equilibrium data for scapolite have been used to calculate co2 activities aco2 and to evaluate the presence or absence of a fluid phase in high grade scapolite bearing meta anorthosite granulites calc silicates and mafix xenoliths the assemblage scapolite plagioclase garnet quartz may be used to calculate or limit aco2 by the reaction meionite quartz grossular anorthite co2 granulites from four high grade terranes grenville province canada sargut belt india furua complex tanzania bergen arcs norway yield aco2 0 4 1 with most 0 7 for scapolite bearing granulites from the furua complex in which aco2 0 9 calculated h2o activities ah2o based on phlogopite dehydration equilibria are uniformly low 0 1 0 2 the aco2 calculated for meta anorthosite from the grenville province ontario ranges from 0 2 to 0 8 for grenville meta anorthosite also containing epidote the ah2o calculated from clinozoisite dehydration ranges from 0 2 to 0 6 calc silicates from the grenville sargur and furua terranes mostly yield aco2 1 the calculated fluid activities are consistent with metamorphism 1 in the presence of a mixed co2 h2o fluid phase in which co2 is the dominant fluid species but other c o h s species are minor 2 in the absence of a bulk fluid phase fluid absent metamorphism or 3 in the presence of a fluid bearing melt phase the results for many granulites and grenville meta anorthosite are consistent with the presence of a co2 rich mixed co2 h2o fluid phase in contrast the relatively restricted and low values of aco2 for calc silicates require an h2o rich fluid or absence of a fluid phase during metamorphism the range of values for xenoliths are most consistent with absence of a fluid phase the primary implication of these results is that a co2 rich fluid accounts for the reduced ah2o in scapolite bearing granulites however scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase and the presence of scapolite is not a priori evidence of a co2 rich fluid phase in addition close association of scapolite free mafic granulites with scapolite bearing granulites having identical mineral compositions in the furua complex and the absence of scapolite from most granulite terranes implies that a co2 rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

excess helium and neon in the southeast pacific tracers for glacial meltwater 1 we analyze helium he and neon ne isotope data sets from the southeast pacific sector of the southern ocean collected in 1992 and 1994 and describe a new method to estimate glacial meltwater fluxes independent of previous approaches the waters sampled during these cruises reveal large atmospheric he and ne excess concentrations in the upper 500 m that are derived predominantly from the dissolution of air contained in glacial ice during melting underneath and at the front of floating ice shelves at elevated hydrostatic pressure on the stations sampled the meltwater fractions are largest near the northern and southern ice shelf fronts of george vi ice shelf 1 5 2 5 consistent with prior inferences from 18o the inventories of 4he and ne with local glacial meltwater origin south of 70 s between 210 e and 295 e are 2 5 0 7 1011 cm3stp 4he and 7 9 2 7 1011 cm3stp ne respectively the associated meltwater volumes are 430 120 gt for 4he and 400 140 gt for ne for an estimated seawater residence time of 2 years on the continental shelf the amounts of glacial meltwater translate into bulk melting rates of 215 170 gt yr 1 for 4he and 200 170 gt yr 1 for ne the values derived from 4he are expected to be larger than those obtained from ne because of the terrigenic he component which accounts for 10 15 of the he excess in this region our regional melting rate of 200 215 gt yr 1 is comparable to a prior estimate but subject to uncertainties resulting from the station distribution the air sea gas exchange and the estimate of the net transport through the study region

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

basal melt and freezing rates from first noble gas samples beneath an ice shelf a climatically induced acceleration in ocean driven melting of antarctic ice shelves would have consequences for both the discharge of continental ice into the ocean and thus global sea level and for the formation of antarctic bottom water and the oceanic meridional overturning circulation using a novel gas tight in situ water sampler noble gas samples have been collected from six locations beneath the filchner ice shelf the first such samples from beneath an antarctic ice shelf helium and neon are uniquely suited as tracers of glacial meltwater in the ocean basal meltwater fractions range from 3 6 near the ice shelf base to 0 5 near the sea floor with distinct regional differences we estimate an average basal melt rate for the filchner ronne ice shelf of 177 95 gt year independently confirming previous results we calculate that up to 2 7 of the meltwater has been refrozen and we identify a local source of crustal helium

crustal velocity structure of the superior and grenville provinces of the southeastern canadian shield the 1992 lithoprobe abitibi grenville seismic refraction experiment was conducted using four profiles across the grenville and superior provinces of the southeastern canadian shield delay time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another with the largest velocity values occurring underneath the central gneiss and the central metasedimentary belts south of the grenville front the grenville front tectonic zone is imaged as a southeast dipping region of anomalous velocity gradients extending to the moho the velocity anomaly maps suggest an archean crust may extend horizontally 140 km beneath the northern grenville province near surface velocity anomalies correlate well with the known geology the most prominent of these is the sudbury structure which is well mapped as a low velocity basinal structure the tomography images also suggest underthrusting of the pontiac and quetico subprovinces

proterozoic granulite facies metamorphism in the southeastern reynolds range central australia geological context p t path and overprinting relationships in the southeastern reynolds range central australia a low p granulite facies metamorphism affected two sedimentary sequences the lander rock beds and the reynolds range group in the context of the whole of the reynolds range and the adjacent anmatjira range this metamorphism is m3 in a sequence m1 4 that occurred over a period of 250 ma in particular m1 affected the lander rock beds prior to the deposition of the reynolds group m3 has an areally restricted high grade area in the southeastern reynolds range affecting both the reynolds range group and the underlying lander rock beds the effects of m3 are characterized by spinel quartz bearing peak metamorphic assemblages in metapelites which imply peak conditions of 750 c and 4 5 1 kbar and involved isobaric cooling or compression with cooling it is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for m3 in the southeastern reynolds range evidence of both the unconformity between the two rock groups and previous metamorphism deformation has been completely erased by recrystallization during m3 d3

generation of pseudotachylite under granulite facies conditions and its preservation during cooling discontinuous ultramylonite zones cut proterozoic granulite facies gneisses in macrobertson land east antarctica and preserve evidence of ductile non coaxial flow and reverse sense of shear cross cutting relationships indicate that ultramylonite deformation involved overthrusting to the east but progressively rotated to involve overthrusting to the north rotation of the principal compressive stress axes is inferred extensive pseudotachylite developed during ultramylonitization the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation neoblastic sillimanite indicates ultramylonitization occurred at 520 c on the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at 700 c and 7 3 0 5 kbar at ah2o 0 3 and low aco2 comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the rayner complex experienced a late increase in pressure of 1 2 kbar during ultramylonitization the p t ah2o conditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates most of the pseudotachylite veins are undeformed the mechanism s of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite further strain having been mostly accommodated by a new or subsidiary shear zone renewed stress at reduced strain rates or renewed stress in zones in which the proportion of pseudotachylite was significantly higher could have led to the rare occurrences of deformed pseudotachylite the preservation of fine grained pseudotachylite is dependent on it remaining dry

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

u pb geochronology of the sublayer environment sudbury igneous complex ontario cu ni sulfide deposits of the sudbury igneous complex are hosted by the inclusion rich sublayer which occurs discontinuously between the footwall and main mass of the sudbury igneous complex by brecciated footwall in the vicinity of the sudbury igneous complex and by offset dikes protruding for tens of kilometers into the country rocks the genesis of mafic ultramafic inclusions in the sublayer and the relationships between the sublayer main mass and offset dikes have long been at the center of debate on sudbury geology this paper reports u pb isotope data on zircon and baddeleyite from samples representing norite matrix melanorite and olivine melanorite pods a metapyroxenite inclusion and diabase from the sublayer at the whistle embayment these samples yield ages ranging from 1848 1 or 1 8 to 1849 1 or 1 1 ma similar to a u pb age of 1849 8 or 2 0 ma obtained for zircons in quartz diorite from the copper cliff offset and indistinguishable from the previously published average age of 1850 or 1 ma for the main mass of the sudbury igneous complex none of the samples yield evidence for inherited zircon components the isotopic data prove that the exotic mafic ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive moderately mg rich magmas that crystallized in the early stages of development of the sudbury igneous complex such mg rich cumulates are unlikely to have crystallized from melted average upper crust

winter mixed layer entrainment of weddell deep water observations from the somov during the us ussr weddell polynya expedition show that the mixed layer below the sea ice just prior to the austral spring retreat in the 60 s greenwich meridian region has an oxygen content of 7 4 ml 1 this is 86 of full oxygen saturation representing an oxygen deficit relative to full saturation of 1 1 ml 1 the source of this deficit is believed to be a consequence of oxygen poor 4 5 ml 1 weddell deep water wdw entrainment by the winter mixed layer assuming effective cutoff of ocean atmosphere oxygen exchange by the nearly complete snow and sea ice cover with no net impact of oxygen content as a result of biological factors a mixing ratio of 1 3 for wdw to beginning of winter surface water is required to explain the end of winter mixed layer oxygen content accompanying the wdw transfer into the mixed layer is heat transfer of approximately 7 103 cal cm2 2 9 108 j m2 during the five winter months of sea ice coverage as well as salt transfer which requires 34 cm of fresh water to produce typical mixed layer salinity during the seven ice free months when entrainment is expected to be minor diffusive heat and salt flux continues a mean annual heat flux of 12 w m2 is suggested with an annual demand for freshwater of 46 cm yr consideration of the winter period salinity budget indicates net sea ice melting of 20 cm which can be attributed to regional convergence of sea ice the remaining freshwater is derived from excess precipitation and possibly iceberg melt oxygen undersaturation of the winter surface water suggests slightly less potential for abyssal water ventilation than might be expected from a fully saturated condition

the distribution of 14c and 39ar in the weddell sea carbon 14 and 39ar data from the weddell sea are presented and discussed values of 14c and 39ar are low in the winter mixed layer 14c 90 to 125 39ar 85 modern these low values are consistent with the surface layer dynamics which is dominated by entrainment of relatively old water of circumpolar origin and reduced gas exchange during sea ice cover the 14c and 39ar values of the deep and bottom waters range from 160 to 150 and 38 to 57 modern respectively the 14c values of weddell sea bottom water wsbw found in the central weddell sea along a 0 longitude section are only slightly higher than those of the overlying weddell sea deep water wsdw showing that the influence of bomb 14c on these waters is small part of the wsbw with higher 14c values observed in the northwestern weddell sea seems to escape through the south sandwich trench and part seems to mix from a boundary current into the central weddell sea the observed 14c distribution is consistent with the hypothesis that ice shelf water isw is a source of wsbw a simple conceptual model of the surface layer dynamics is used to estimate the prebomb 14c values of surface water and winter water to be about 140 and 130 respectively using mixing ratios between wsdw and shelf water derived from temperature salinity and 3he data the prebomb 14c values of wsbw are estimated to be 157 potential temperature of wsbw 0 7 c the 39ar concentration of wsbw with a potential temperature of 0 7 c is determined to be 57 modern bomb radiocarbon water column inventories are estimated and discussed

a numerical model of the weddell sea large scale circulation and water mass distribution a circumpolar model for regional studies of the wind driven and thermohaline circulation of the southern ocean including the major sub ice shelf areas is described a first series of numerical experiments focusing on the weddell sea reveals a pronounced and persistent double cell structure of the weddell gyre with a maximum transport of 60 sv in agreement with observations experiments with artificial passive tracers point to the shallow shelf areas off the filchner ronne ice shelf as main locations for bottom water production the trajectories of lagrangian floats are used to determine the pathways and timescales of water mass spreading in the model in addition the effect of sub ice shelf forcing on the water mass characteristics is evaluated it is shown that water modified in the sub ice cavities contributes significantly to the deep and bottom water formation along the continental slope and affects the water mass characteristics throughout the weddell sea by increasing the stability of the near surface stratification and preventing deep convection

export of weddell sea deep and bottom water an extensive set of conductivity temperature depth ctd lowered acoustic doppler current profiler ladcp data obtained within the northwestern weddell sea in august 1997 characterizes the dense water outflow from the weddell sea and overflow into the scotia sea along the outer rim of the weddell gyre there is a stream of relatively low salinity high oxygen weddell sea deep water defined as water between 0 and 0 7 c constituting a more ventilated form of this water mass than that found farther within the gyre its enhanced ventilation is due to injection of relatively low salinity shelf water found near the northern extreme of antarctic peninsula s weddell sea shelf shelf water too buoyant to descend to the deep sea floor the more ventilated form of weddell sea deep water flows northward along the eastern side of the south orkney plateau passing into the scotia sea rather than continuing along an eastward path in the northern weddell sea weddell sea bottom water also exhibits two forms a low salinity better oxygenated component confined to the outer rim of the weddell gyre and a more saline less oxygenated component observed farther into the gyre the more saline weddell sea bottom water is derived from the southwestern weddell sea where high salinity shelf water is abundant the less saline weddell sea bottom water like the more ventilated weddell sea deep water is derived from lower salinity shelf water at a point farther north along the antarctic peninsula transports of weddell sea deep and bottom water masses crossing 44 w estimated from one ladcp survey are 25 106 and 5 106 m3 s 1 respectively the low salinity better ventilated forms of weddell sea deep and bottom water flowing along the outer rim of the weddell gyre have the position and depth range that would lead to overflow of the topographic confines of the weddell basin whereas the more saline forms may be forced to recirculate within the weddell gyre

component separation of oceanic helium a new procedure to quantify the components of oceanic helium terrigenic 3he and 4he released from the ocean floor and tritiugenic 3he from tritium decay is described terrigenic he and nonatmospheric 3he i e terrigenic and tritiugenic combined are obtained in terms of measured concentrations of the he isotopes and also of neon ne which improves the separation considerably assuming terrigenic he to vanish in the mixed layer for the subsequent separation of terrigenic and tritiugenic 3he additional information is required and 3he due to natural tritium represents a complication the procedure is applied to data from a hydrographic section in the south atlantic 19 s 1991 and one in the eastern mediterranean 1987 the 1 data precisions and a systematic error accounting for uncertainties in mixed layer he are approximately 0 3 sections of the new representations of oceanic he and 3he and comparisons to the nearest classical quantities i e 3he he are presented in the south atlantic the 3he distribution reflects the hydrographic structure east of 20 w the average 3he 4he ratio of terrigenic he below 800 m is 4 5 0 8 times the atmospheric ratio which implies a substantial contribution of crustal he in the upper waters tritiugenic 3he 0 5 tritium units 20 is separated from terrigenic 3he in the eastern mediterranean tritiugenic 3he is quantified throughout the water column in the presence of substantial levels of terrigenic he the release rate of terrigenic he from the sea floor is found to be 3 1 1 2 1010 atoms m 2 s 1 similar to the rate for continental crust with a mantle he contribution of 5 1 2 only recommendations for future work are to reduce the mentioned systematic error and the uncertainty margins of the he and ne solubilities and of 3he due to natural tritium

the distribution of tritium and cfcs in the weddell sea during the mid 1980s abstract transient tracer data tritium cfc11 and cfc12 from the southern central and northwestern weddell sea collected during polarstern cruises ant iii 3 ant v 2 3 4 and during andenes cruise nare 85 are presented and discussed in the context of hydrographic observations a kinematic time dependent multi box model is used to estimate mean residence times and formation rates of several water masses observed in the weddell sea ice shelf water is marked by higher tritium and lower cfc concentrations compared to surface waters the tracer signature of ice shelf water can only be explained by assuming that its source water mass western shelf water has characteristics different from those of surface waters using the transient nature of tritium and the cfcs the mean residence time of western shelf water on the shelf is estimated to be approximately 5 years ice shelf water is renewed on a time scale of about 14 years from western shelf water by interaction of this water mass with glacial ice underneath the filchner ronne ice shelf the ice shelf water signature can be traced across the sill of the filchner depression and down the continental slope of the southern weddell sea on the continental slope new weddell sea bottom water is formed by entrainment of weddell deep water and weddell sea deep water into the ice shelf water plume in the northwestern weddell sea new weddell sea bottom water is observed in two narrow deep boundary currents flowing along the base of the continental slope classically defined weddell sea bottom water 0 7 c and weddell sea deep water 0 7 c 0 c are ventilated from the deeper of these boundary currents by lateral spreading and mixing model based estimates yield a total formation rate of 3 5sv for new weddell sea bottom water 1 0 c and a formation rate of at least 11sv for antarctic bottom water 0 5 c

large scale mass transports water mass formation and diffusivities estimated from world ocean circulation experiment woce hydrographic data 1 a global inversion is used to estimate the oceanic mass transports and vertical diffusivities based on a subset of hydrographic data from the world ocean circulation experiment woce and java australia dynamic experiment the analysis is based on the linear inverse box model of ganachaud and wunsch 2000 that consistently combines the transoceanic sections a globally consistent solution is obtained for a depth independent adjustment to the thermal wind field the freshwater flux divergences the ekman transport and the advective and diffusive dianeutral fluxes between layers a detailed error budget permits calculation of statistical uncertainties taking into account both the nonresolved part of the solution and the systematic errors due to the temporal oceanic variability the horizontal structure of the transports for different density classes is analyzed regionally during the woce period 1985 1996 net meridional transports are estimated with 16 2 sv 106 m3 s of north atlantic deep water nadw being produced in the northern north atlantic moving southward entraining antarctic bottom water aabw and antarctic intermediate water increasing in volume transport to 23 3 sv at 30 s in the southern ocean 21 6 sv of bottom water was formed from lower circumpolar deep water which corresponds approximately to the lower nadw density range bottom water inflows nadw aabw mixture to the atlantic indian and pacific oceans are estimated at 6 1 3 11 4 and 7 2 sv respectively in the indian and pacific oceans this water returns southward at deep and intermediate levels property anomaly conservation constraints permit estimation of dianeutral diffusivities in deep layers with a global average of 3 7 0 7 cm2s 1 north of 30 s

low pressure granulite facies metapelitic assemblages and corona textures from macrobertson land east antarctica the importance of fe2o3and tio2in accounting for spinel bearing assemblages low pressure granulite facies metasedimentary gneisses exposed in macrobertson land east antarctica include hercynitic spinel bearing metapelitic gneisses peak metamorphic mineral assemblages include spinel rutile ilmenite sillimanite garnet spinel ilmenite sillimanite garnet cordierite ortho pyroxene magnetite ilmenite garnet spinel cordierite biotite ilmenite and orthopyroxene cordierite biotite each with quartz k feldspar and melt the presence of garnet biotite and cordierite orthopyroxene bearing assemblages implies crossing tie lines in afm projection for the k2o feo mgo al2o3 sio2 h2o kfmash system this apparent contradiction and the presence of spinel rutile and ilmenite in the assemblages is acounted for by using the kfmash tio2 o2 system i e afm tio2 fe2o3 we derive a petrogenetic grid for this system applicable to low pressure granulite facies metamorphic conditions retrograde assemblages are interpreted from corona textures on hercynitic spinel and fe ti oxides the relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

mafic intrusions in the footwall of the sudbury igneous complex origin of the sudbury impact melt sheet and its associated ore deposits abstract the majority of the mafic intrusions and volcanic rocks in the footwall of the sudbury igneous complex sic which is believed to have formed from a melt sheet produced by the impact of a very large bolide belong to one of two large mafic igneous suites viz the 2450 ma east bull lake ebl suite of intrusions or the 2200 ma nipissing diabase suite of intrusions this has been established by u pb la icp ms dating of zircons from the newly recognized frood intrusion which hosts the frood and stobie ore bodies and two of the sudbury gabbro intrusions and comparison of the geochemistry of these intrusions with that of the ebl intrusive suite and the nipissing diabase suite the age of the frood intrusion 2421 32 ma falls within error of other ebl type intrusions whereas the ages of the nairn 2203 11 ma and totten 2168 11 ma intrusions fall within error of published ages for the nipissing diabase moreover the primitive mantle normalized patterns of the petrogenetic trace elements on extended spidergrams of the nairn and totten sudbury gabbros are similar to those of the nipissing diabase as are those of the frood intrusion to other members of the ebl intrusive suite the ni cu pge sulfide deposits at sudbury are associated with the lower contact of the sic where a discontinuous inclusion rich norite contains much of the mineralization at the contact and an inclusion bearing quartz diorite is directly associated with the mineralization of the radial and concentric offset dykes the sublayer rocks and the overlying unit of melanocratic norites provide strong evidence for a relationship between the ore deposits and both inclusions and melts of mafic ultramafic target rocks the most common pre impact mafic ultramafic target rocks are intrusions of the ebl and nipissing diabase suites although there are also archean aged mafic ultramafic enclaves in the gneisses beneath the north range of the sic the totten orebody in the worthington offset and the kelly lake ore body in the copper cliff offset are directly associated with footwall rocks comprising sudbury gabbro and nipissing gabbro respectively whereas the frood stobie offset and the associated frood and stobie ore deposits are sandwiched in a breccia belt containing large rafts of amphibole megacrystic gabbro belonging to the extensively broken up frood intrusion olivine bearing mafic and ultramafic rocks some of which exhibit shock metamorphic features but others do not are common inclusion types and occur in diverse sublayer environments the unshocked melanorite and olivine melanorite inclusions share common geochemical characteristicsde indicating that they were derived from a similar source their geochemistry is also similar to that of the frood intrusion as well as the east bull lake intrusion this indicates that they may have been sourced from east bull lake type intrusions that were incorporated into the melt sheet at the time of melt generation mass balance calculations indicate that the felsic country rocks which account for 77 of the total volume of the sic and contain 18 ppm ni and 21 ppm cu together with the sulfide poor mafic country rocks which account for 23 of the sic and contain 133 ppm ni and 92 ppm cu have too little ni and cu to account for the ni and cu contents of the quartz diorite the composition of which provides an indication of that of the initial melt sheet calculations using the r factor equation of campbell and naldrett 1979 indicate that the magmas that formed the norites and the ore deposits had 137 ppm ni and 121 ppm cu mass balance calculations have been used to determine if the country rocks have sufficient ni and cu in them to generate the ore deposits the weighted median ni content of the sulfide poor

structural constraints on the proterozoic reworking of archaean crust in the rayner complex macrobertson and kemp land coast east antarctica abstract the high grade metamorphic rocks exposed along the macrobertson and east kemp land coasts consist of in the west archaean gneisses reworked during a ductile mid proterozoic deformation and in the east foliated proterozoic charnockite gneiss containing large remnants of gneiss of probable archaean age evidence for three folding events is recognized in the reworked gneisses an early recumbent terrain is deformed by two upright events possessing near vertical axial surfaces the charnockite gneiss preserves evidence of only the final event these three events comprise the rayner structural episode which was a consequence of the ductile thrusting of archaean gneiss westward toward a stable archaean block the napier complex kinematic indicators are consistent with formation of the proterozoic recumbent terrain by dominantly shear deformation dissection of both the rayner and napier complexes by retrograde shear zones and mylonite pseudotachylite zones was a consequence of the northward thrusting of the two complexes during the cambrian

deep and bottom water of the weddell sea s western rim oceanographic observations from the ice station weddell 1 show that the western rim of the weddell gyre contributes to weddell sea bottom water a thin 300 meters highly oxygenated benthic layer is composed of a low salinity type of bottom water overlying a high salinity component this complex layering disappears near 66 s because of vertical mixing and further inflow from the continental margin the bottom water flowing out of the western rim is a blend of the two types additionally the data show that a narrow band of warmer weddell deep water hugged the continental margin as it flowed into the western rim providing the continental margin with the salt required for bottom water production

the contribution of the weddell gyre to the lower limb of the global overturning circulation the horizontal and vertical circulation of the weddell gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports the gyre is found to convey 42 8 sv 1 sv 106 m3 s 1 across the central weddell sea and to intensify to 54 15 sv further offshore this circulation injects 36 13 tw of heat from the antarctic circumpolar current to the gyre and exports 51 23 msv of freshwater including 13 1 msv as sea ice to the mid latitude southern ocean the gyre s overturning circulation has an asymmetric double cell structure in which 13 4 sv of circumpolar deep water cdw and relatively light antarctic bottom water aabw are transformed into upper ocean water masses by mid gyre upwelling at a rate of 2 2 sv and into denser aabw by downwelling focussed at the western boundary 8 2 sv the gyre circulation exhibits a substantial throughflow component by which cdw and aabw enter the gyre from the indian sector undergo ventilation and densification within the gyre and are exported to the south atlantic across the gyre s northern rim the relatively modest net production of aabw in the weddell gyre 6 2 sv suggests that the gyre s prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of indian sourced aabw

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

the effect of dynamic recrystallization on olivine crystal preferred orientations in mantle xenoliths deformed under varied stress conditions abstract spinel peridotite xenoliths from persani mountains southeastern carpathians romania were submitted to deformation by dislocation creep accompanied by dynamic recrystallization under variable stress and temperature conditions predominance of low angle boundaries parallel to 100 with well defined 0vw rotation axes in olivine indicates that subgrain rotation is the main recrystallization mechanism and that dislocation glide occurs mainly in 100 0kl systems analysis of olivine crystal preferred orientations highlights that recrystallization results in dispersion of the orientations of recrystallized grains relatively to the parent grains this dispersion may be quantified by a dimensionless dispersion factor defined as the ratio of the random component of the 100 axis distributions between the recrystallized grains and porphyroclasts this factor is largely independent from the overall fabric strength the number of grains analyzed and the recrystallized grain size the olivine cpo strength and anisotropy at the rock scale quantified by j index are largely controlled by the volume of recrystallized grains which depends on finite strain comparison of olivine cpo in coarse and fine grained porphyroclastic peridotites shows that deviatoric stress and temperature conditions play nevertheless a role on the cpo evolution during recrystallization selective grain growth which is favored at high temperature and low stresses does counteract the dispersion produced by the nucleation processes

apparent loss of cfc 113 in the upper ocean we report and evaluate data of the chlorofluorocarbon cfc 113 in comparison with concurrent data for cfc 12 from cruises into the temperate north atlantic the tropical western pacific the eastern mediterranean and the weddell sea we consistently find cfc 113 deficiencies in the warmer upper waters which we interpret as cfc 113 depletion at rates of the order of 3 per year with possibly accelerated rates in the mixed layer or near the surface these results severely limit a tracer utility of cfc 113 in the warm upper waters of the ocean tracer applications in the deep waters of the ocean should be much less affected since cfc 113 loss in such waters appears to be far slower this also holds for the eastern mediterranean despite its rather warm deep waters for the weddell sea the data indicate a cfc 113 deficiency 20 in the shelf waters which converts into a similar deficiency in newly formed deep and bottom water such cfc 113 deficiency has to be accounted for in southern ocean tracer work future work is proposed to study cfc 113 loss in the ocean in additional detail and to elucidate the loss mechanism s on which our data give little clue

thermohaline stratification below the southern ocean sea ice the end of winter stratification within the cold cyclonic trough of the weddell gyre near 60 s between 5 e and the greenwich meridian is resolved with the mikhail somov data set the temperature maximum of the weddell deep water wdw is for the most part less than 0 5 c but warmer cells of wdw are found these warm wdw cells have temperature salinity and oxygen properties similar to the wdw characteristic of the weddell gyre inflow which is situated to the southeast of the mikhail somov study region the warm wdw cells are accompanied by domes in the pycnocline of 40 m amplitude over the surrounding pycnocline while deeper isopycnals are depressed anticyclonic shear below the 27 83 isopycnal within the warm wdw cells is compensated by the cyclonic shear associated with the pycnocline dome the pycnocline domes are exposed to about 50 greater entrainment by the turbulently active winter mixed layer relative to the regional entrainment rate this entrainment can significantly erode the warm cells in a single winter season introducing excess heat and salt into the mixed layer while the heat is lost to the atmosphere the excess salt is not necessarily compensated by increased fresh water introduction it is hypothesized that the warm wdw cells within the weddell gyre trough are derived from instability within the frontal zone which extends from maud rise to the northeast separating the weddell warm regime from the cold regime greater than normal injection of warm wdw cells into the weddell gyre trough would increase the surface salinity which would tend to destabilize the pycnocline increasing the probability of deep convection and polynya events

interannual variability and ventilation timescales in the ocean cavity beneath filchner ronne ice shelf antarctica 1 multiyear time series of ocean current and temperatures from beneath filchner ronne ice shelf antarctica demonstrate both seasonal and interannual variability the seasonal signal is visible at all measurement sites although it was swamped for a 2 year period 1999 2001 when extraordinarily light sea ice cover in the southern weddell sea during the 1997 1998 austral summer caused an anomalously large pulse of high salinity shelf water to flush beneath the ice shelf the pulse was observed twice at an instrumented site near the berkner island coast once on its way to the filchner depression and once after the signal had propagated around the depression and returned to the site as an anomalously large pulse of ice shelf water the timings of the signal allow an estimate of 24 30 months for the flushing timescales of the sub ice shelf ocean cavity indicating that the cavity is highly responsive to external forcing a timescale for the full ventilation of the cavity of 4 5 years is obtained from the length of time the sub ice shelf conditions take to return to their original state a timescale significantly shorter than previous estimates

southern ocean winter mixed layer austral winter 1986 observations from the polarstern along the greenwich meridian from the ice edge to the antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation incorporation of weddell deep water wdw into the winter mixed layer responsible for this undersaturation also introduces heat and salinity into the surface layer which strongly influences the mixed layer sea air exchanges and sea ice formation processes the total wdw transfer into the mixed layer averages 45 myr 1 implying a residence time for the surface water of 2 5 years the associated winter heat flux is 41 w m 2 which limits ice thickness to about 0 55 m agreeing quite well with observations the air temperatures during the cruise are just sufficient to remove the wdw heat input in the presence of observed ice thickness and concentration this suggests that the sea ice cover and wdw heat input into the mixed layer are in approximate balance by midwinter the annual heat flux from wdw to the surface layer and hence into the atmosphere is estimated as 16 w m 2 extrapolation of the greenwich meridian wdw entrainment value to the full circumpolar 60 70 s belt yields total up welling of 24 106 m3 s 1 similar extrapolation of the heat flux value gives a circumpolar total of 2 8 1014 w as a consequence of circulation topography interaction the maud rise water column stands out as an anomaly relative to the surrounding region with a significantly more saline and dense mixed layer below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m this uplifting is believed to be a response of the upstream flow encountering the rise increased upstream flow would be expected to increase maud rise upwelling and the dependent salinity density of the mixed layer slight increases in the mixed layer density could trigger a convective mode and generation of a polynya it is hypothesized that spin up of the weddell gyre s barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over maud rise

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

notes on southern ocean hydrography sea ice and bottom water formation abstract the southern ocean circulation and sea ice distribution is briefly described the formation of extremely cold bottom water in the weddell sea and its relation to the floating ronne filchner ice shelves is discussed it is shown that a concentrated swift eroding bottom current with anomalous low 18 o ratio transports the cold and dense ice shelf water from the shelf towards large depths comments are made on possible implications of this process for the large scale deep water circulation and for the interpretation of sediment cores

oxygen 18 and helium as tracers of ice shelf water and water ice interaction in the weddell sea oxygen 18 and helium isotope data from stations located in the filchner depression on the continental slope of the southern weddell sea and in the central weddell sea are presented and discussed the 18o and 4he signals imprinted on the water circulating under the filchner ronne ice shelf ice shelf water or isw due to melting of glacial ice at the base of the ice shelf are traced across the sill separating the filchner depression from the weddell sea low 18o values are correlated with high 4he concentrations in the isw found in the filchner depression minimum 18o values 0 8 maximum 4he concentrations about 4 7 10 8 cm3 stp g 1 the fraction of glacial meltwater contained in the isw found in the filchner depression is estimated to about 6 to 7 the 18o and helium isotope data from the overflowing shelf water component observed on the continental slope confirm the hypothesis that isw contributes significantly to the weddell sea bottom water wsbw on the basis of a inultiparameter water mass analysis it is discussed which fraction of the wsbw originates from isw and which other shelf waters potentially could contribute to wsbw

pseudotachylyte and the development of ultramylonite bands in the saint barthelemy massif french pyrenees abstract in mylonitic gneisses of the saint barthelemy massif in the french pyrenees evidence was found that ultramylonite bands developed by ductile deformation of pseudotachylyte in the gneiss a mylonitic shape fabric was produced by a continuous structural event during retrogression from amphibolite to upper greenschist facies decrease in temperature caused gradual hardening in the gneiss which led to the development of pseudotachylyte bands by intermittent seismic slip these acted as weak zones in which ductile deformation was concentrated a microstructure typical of ultramylonite was produced in these deformed pseudotachylyte bands

ice shelf water overflow and bottom water formation in the southern weddell sea 1 cold shelf waters flowing out of the filchner depression in the southern weddell sea make a significant contribution to the production of weddell sea bottom water wsbw a precursor to antarctic bottom water aabw we use all available current meter records from the region to calculate the flux of cold water 1 9 c over the sill at the northern end of the filchner depression 1 6 0 5 sv and to determine its fate the estimated fluxes and mixing rates imply a rate of wsbw formation referenced to 0 8 c of 4 3 1 4 sv we identify three pathways for the cold shelf waters to enter the deep weddell sea circulation one path involves flow constrained to follow the shelf break the other two paths are down the continental slope resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36 w and 37 w mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m probably aided by thermobaricity the plume water at this depth can flow at a speed approaching 1 m s 1 implying that the flow is occasionally supercritical we postulate that such supercriticality acts to limit mixing between the plume and its environment the transition from supercritical to slower more uniform flow is associated with very efficient mixing probably as a result of hydraulic jumps

baroclinic instability of density current along a sloping bottom and the associated transport process numerical experiments with a three dimensional nonhydrostatic ocean model have been carried out to investigate the dynamic processes of downslope density current and associated transport of dense water focusing on the effects of bottom slope for the range of slope inclination s from 0 to 0 04 bottom slope has two effects on baroclinic instability of the flow one is the stabilizing effect due to the topographic effect and to offshoreward increase in water depth the other is the destabilizing effect due to steepened inclination of isopycnal surface which increases available potential energy since the destabilizing effect over the upper slope overcomes the stabilizing one in the early stage unstable waves develop more rapidly with increasing s nonlinear energy conversion due to reynolds stresses also affects the flow stability and then the growth rate of instability increases linearly with s in this stage after the unstable waves have finite amplitudes in the mature stage the stabilizing effect of bottom slope becomes dominant especially over the lower slope thus vigorous unstable eddies are confined to the upper slope region in steep slope cases while they cover the whole slope region in gentle slope cases and this difference in the eddy field causes a drastic change in downslope transport of dense water in gentle slope cases efficiency of the transport becomes higher with increasing s since the destabilizing effect overcomes the stabilizing one as a whole on the contrary the efficiency becomes lower with increasing s in steep slope cases since the stabilizing effect surpasses the destabilizing one as a result the most effective transport is realized in the case with s 0 005 although the transport due to eddies in steep slope cases becomes weak over the lower slope it is still larger than that due to the bottom ekman current since the eddy transport occurs in the bottom layer with a thickness of 100 200 m which is 4 8 times the ekman layer thickness