nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

nd isotope systematics of coarse and fine grained sediments examples from the middle proterozoic belt purcell supergroup sm nd isotopic compositions are reported for 20 fine and 5 coarse grained samples of the belt purcell supergroup of western north america varepsilon nd and sm nd ratios for fine grained belt purcell sediments are remarkably uniform indicating crustal residence ages ranging only from 1 6 to 2 1 ga varepsilon nd and sm nd ratios for coarse grained sediments however are much more varied and contain evidence of sm nd fractionation during sedimentary recycling assuming that this ree fractionation occurred between the time source areas were eroded and when sediment was deposited a new method of calculating crustal residence age intervals is derived for the coarse grained samples which serves to place constraints on the average time since source materials were extracted from the mantle the nd data from both sample groups provide an average age estimate of 2 0 pm 0 3 ga for the middle proterozoic crust from which most of the belt purcell supergroup was derived the isotopic data toge

ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

geochemical mineralogical and isotopic data relating to the origin and tectonic setting of the rossland volcanic rocks southern british columbia bulk rock and mineral chemical and isotopic analyses of rossland volcanic rocks are used to infer the nature of the magma extruded in the nelson rossland area of southern british columbia during th

nd sr isotopic constraints on the interactions of the intermontane superterrane with the western edge of north america in the southern canadian cordillera sr and nd isotopic compositions of the late paleozoic metavolcanics and late triassic to early tertiary granitoids from four magmatic episodes in the southern canadian cordillera from the kootenay arc to the fraser fault have been used to i identify the sources of these rocks ii constrain the compressive tectonic history from middle jurassic to paleocene and iii constrain the western boundary of the basement in this region the 215 190 ma old primitive granitoids end 3 1 to 8 7 87sr 86sr 0 7028 0 7043 of the late triassic and early jurassic magmatic episode were emplaced in the paleozoic oceanic crust of quesnellia end 2 9 to 9 3 prior to its obduction over the basement in contrast during the younger magmatic episodes middle late jurassic cretaceous and early tertiary the granitoids from western quesnellia show primitive isotopic compositions and those from eastern quesnellia show eastward increasing crust contaminated compositions the contaminated characters of the middle

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

north american margin origin of quesnel terrane strata in the southern canadian cordillera inferences from geochemical and nd isotopic characteristics of triassic metasedimentary rocks new geochemical and nd isotopic data from triassic ca 220 ma clastic metasedimentary rocks assigned to the quesnel terrane quesnellia in southeastern british columbia show that detritus was derived from both primitive and evolved sources twenty four samples yield end 2 2 0 values between 9 1 and 5 6 independent of sample location these samples have eu anomalies between 0 6 and 1 0 la n yb n ratios between 3 8 and 11 4 la sc ratios ranging from 0 38 to 4 76 and th sc ratios between 0 09 and 1 62 this range of isotopic and geochemical values overlaps that of samples from upper paleozoic to mesozoic rocks of the north american miogeocline and upper triassic nicola group volcanic rocks of the quesnel terrane the range in nd isotopic and geochemical values supports the interpretation that detritus was derived from both continental north america evolved cratonic source and a volcanic arc primitive source developing on the continental margin these data do not support previous interpretations of the quesnel terrane as an intraoceanic arc the inferred depositional link between north america and the quesnel arc in triassic time is ca 40 ma older than the hypothesized obduction of the quesnel terrane onto the north american continental margin and when combined with existing geological information strongly supports a pericratonic nonexotic origin for the quesnel terrane in southeastern british columbia

erratum superimposed low grade metamorphism in the mount fisher area southeastern british columbia implications for the east kootenay orogeny middle proterozoic 1500 1350 ma belt purcell strata exposed in the purcell and southwestern rocky mountains were affected by at least three distinct episodes of deformation and regional metamorphism the oldest episode 1300 1350 ma apparently terminated belt purcell sedimentation and involved foiding regional metamorphism and granitic intrusion the second episode 800 900 ma occurred during deposition of the windermere supergroup and involved uplift block faulting and low grade regional metamorphism mesozoic cenozoic metamorphism deformation and plutonism overprinted the results of the earlier deformation and metamorphism illite crystallinity and muscovite polymorph ratios indicate that purcell strata in the mount fisher area are in the lower greenschist to prehnite pumpellyite facies of regional metamorphism in the steeples and fisher blocks this metamorphism is related to structures that formed during the late cretaceous paleocene deformation however in the sand creek block the regional metamorphism is related to the development of a spaced cleavage that is folded by a late cretaceous pdeocene nappe regional considerations suggest that this cleavage formed during the 1300 1350 ma episode of deformation and metamorphism the east kootenay orogeny as currently defined embraces the two older episodes of tectonism it is proposed that the term east kootenay orogeny be restricted to designate the 1300 1350 ma episode and that the term goat river orogeny designate the 800 900 ma episode of tectonism the east kootenay and goat rivet orogenies appearto be correlative with the racklan and hayhook orogenies recognized in the northern canadian cordillera

upper crustal recycling in southern britain evidence from nd and sr isotopes analyses of representative rocks from southern britain are used to examine in detail the evolution of this segment of continental crust the vast majority of late precambrian basement rocks have unradiogenic 87sr 86sr ratios and model tdmnd ages of 1 3 ga although locally the rosslare complex and rushton schists have model tdmnd ages 1 7 ga identifying for the first time an old component in the basement of southern britain following the closure of the iapetus ocean sediments exhibit a progressive increase in model tdmnd ages 1 35 ga in the lower palaeozoic to 1 75 ga in the jurassic this variation demonstrates the southerly transport of detritus tdmnd 2 0 ga from the mixed archaean proterozoic terrains of northern europe and america the granitoids have significantly higher end than those of contemporaneous sediments and thus yield a different nd isotope evolution curve when considered in isolation the majority of rocks analysed have model tdmsr ages in the range 1 2 to 0 46 ga with youngest sediments generally recording the lowest model ages therefore with time the sediments record a striking increase in the ratio of model nd to model sr age the data suggest that the rb sr ratios of the crustal rocks is increased by a factor of approximately 2 every 900 ma implying that on average the british upper crust was reworked by erosion and sedimentation every 600 ma

cordilleria revisited with a three dimensional model for cretaceous tectonics in british columbia the cordilleria microcontinent theory for the tectonics of british columbia in the cretaceous and early tertiary and for the origin of the canadian rocky mountains and related features is reviewed new isotopic geological and paleomagnetic data support the hypothesis that the amalgamated superterranes of british columbia and alaska were not attached to the mainland of north america in the early cretaceous and were probably well south of their present relative position consideration of that evidence together with sedimentological evidence from the albian leads to a slightly earlier best estimate 115 ma for the time of initial collision in the yukon new geophysical data for southeastern british columbia and idaho lead to a comparison with the iapetus suture in northern england it is suggested that the cordilleria suture dips west from the present purcell thrust and related structures and should cut the moho a new model for regional deformation in se british columbia is proposed involving a flake

geochemistry and sr nd isotopic composition of eocene lamphrophyre dykes southeastern british columbia mineral compositions geochemical analyses and sr nd isotopic compositions are reported for alkaline and calc alkaline lamprophyres collected along the southern margin of the valhalla complex southeastern british columbia the lamprophyres were emplaced during eocene extension and lithospheric thinning associated with tectonic denudation of the valhalla complex sio2 contents range from 44 4 51 6 wt k2o from 1 3 3 7 wt and volatile contents h2o co2 so3 from 0 8 4 6 wt mgo and cr contents are 9 5 7 6 wt and 540 130 ppm respectively for samples with mg s between 0 69 and 0 65 chrondrite normalized rare earth element patterns are strongly fractionated with cen 120 375 and ybn 8 4 12 7 alkaline lamprophyres contain biotite kaersutite calcic plagioclase and exhibit a limited range in initial 87sr 86sr 0 7051 0 7057 initial end 3 7 to 4 3 and tdm 766 796 ma calc alkaline lamprophyres contain f rich phlogopite and sodic plagioclase and exhibit a wider range in initial 8

nd evidence for proterozoic crustal development in the belt purcell supergroup a knowledge of the chemical and isotopic characteristics of the continents as they existed in the past is essential to an understanding of their evolution1 7 vestiges of these characteristics remain in the sedimentary record for example the character and crustal residence age4 of ancient continental crust has been deduced from the sm nd isotopic patterns of clastic sediments that have survived to the present day4 6 8 we report here sm nd isotopic measurements from the belt purcell supergroup of western north america which form the first detailed isotopic investigation of a major proterozoic sedimentary succession the belt purcell has many features also seen in the detrital load of the world s major contemporary rivers including exceptionally homogeneous 147sm 144nd and 143nd 144nd ratios and a small range of crustal residence ages mantle additions to the continental crust seem to have been relatively minor since the middle proterozoic

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

proterozoic mantle under quesnellia variably reset rb sr mineral isochrons in ultramafic nodules carried up in cenozoic volcanic vents of the southern omineca belt handpicked acid washed diopside orthopyroxene and olivine separates from 14 lherzolite nodules from four localities in southern british columbia have been analyzed for 87rb 86sr and 87sr 86sr the host lavas of late cenozoic to pleistocene age are nepheline normative alkali basalt and basanite with sr and nd isotopic ratios 0 7025 0 7029 0 51310 0 51326 close to those of mid ocean ridge basalt the present day sr isotopic compositions of nodule diopsides 9 239 ppm sr are 0 7021 0 7041 similar to those of the host rocks orthopyroxene and olivine however have 87rb 86sr ratios of 0 1 to 1 0 and much more radiogenic sr 87sr 86sr commonly 0 708 0 720 and in several cases the three minerals form linear arrays in 87rb 86sr versus 87sr 86sr plots if interpreted as mineral isochrons these give dates from 99 to 1989 ma many cluster around 440 770 ma four nodules from two localities west kettle river and big timothy mountain give dates of 1201 1989 ma maximum dates for the other localities

event reconstruction through bayesian chronology massive mid holocene lake burst triggered large scale ecological and cultural change precise timing of natural and cultural events provides a foundation for understanding how past natural phenomena have driven changes in population and culture in this study we used high resolutio

subsidence analysis of the cordilleran miogeocline implications for timing of late proterozoic rifting and amount of extension tectonic subsidence of the cordilleran miogeocline in the western united states during late proterozoic through early paleozoic time is best explained by rifting and subsequent thermal contraction of the lithosphere the tectonic component of subsidence can be calculated by removing the isostatic effects of sediment and water loading the resultant subsidence curves indicate that rifting began about 600 m y ago and that extension of the lithosphere during rifting was in excess of a factor of 1 3

gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the krishna godavari basin offshore india abstract the national gas hydrate program expedition 01 cored ten sites in the krishna godavari basin located on the southeastern margin of india a comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin in situ diagenetic and metabolic reactions and fluid migration and flow pathways gas hydrate was present at all of the sites cored and in situ microbial methanogenesis leads to estimates of depth integrated average gas hydrate saturations that are typically

source terranes for proterozoic sedimentary rocks in southern british columbia nd isotopic and petrographic evidence quartz arenites of the fort steele formation the lowest exposed stratigraphic unit of the purcell supergroup have nd crustal residence times tcr of 2 61 2 63 ga these overlap the low end of the tcr range of the polymetamorphic kenoran hudsonian basement of southern alberta and southwestern saskatchewan the tcr values of overlying fine grained purcell rocks average 2 0 ga near lake windermere the basal toby conglomerates of the windermere supergroup apparently derived from underlying purcell strata have tcr values of 1 98 and 2 03 ga stratigraphically higher argillite and arkoses range from 2 35 to 2 54 ga this higher range indicates contributions from a source terrane different from the main body of the purcell supergroup our interpretation of events in the east kootenay area is that early purcell sedimentation fort steele formation was derived from reworking of the crystalline basement of the alberta shelf as the beltian sea transgressed as the basin deepened it was filled by fine grained

nd sr isotope systematics of nicola group volcanic rocks quesnel terrane volcanic rocks of the nicola group belong to an arc built on the western margin of the quesnel terrane in the late triassic to early jurassic low k high k compositional types define a rb sr isochron of 222 15 ma with initial 87sr 86sr 0 70367 2 the corresponding nd isotopic compositions of these samples end 222 ma 5 1 to 7 8 fall within the range for early mesozoic island arcs a comparable range of end 222 ma 5 0 to 7 9 in picrite shoshonite samples precludes generation of increasingly potassic magmas by progressive metasomatism of the mantle wedge alone source region heterogeneity possibly imparted by changes in the composition of subducted slab components or interaction with amphibole or phlogopite in the source remnant of an earlier permian arc on the quesnel terrane is required to account for geochemical differences between these rock suites crustal contamination is severely limited from the high end values such that continental basement now underlying the quesnel terrane

comparison of isotopic and petrographic provenance indicators in sediments from tertiary continental basins of new mexico abstract isotopic variations of nd in sedimentary rocks provide a new tool for provenance determination we have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two tertiary basins of the rio grande rift north central new mexico the siliciclastic sediments in these basins were derived from varying proportions of precambrian basement and oligocene volcanic rocks which are isotopically and petrographically distinct in the analyzed sediments a good correspondence exists between petrographic and isotopic provenance indicators the data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology the similar sm nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of sm and nd occurs during the formation of the sediments thus the nd model ages of the sediments preserve the crust formation age of the source the sr isotopic and concentration data show trends generally similar to the nd data but they do not indicate the sedimentary provenance as clearly this ambiguity is a consequence of strong rb sr source heterogeneities and rb sr fractionation during the sedimentary cycle

nd evidence for extensive archean basement in the western churchill province canada to determine the extent of reworked archean crust in the western churchill province we have examined sm nd crustal residence ages of basement cores in southern alberta and southwestern saskatchewan along with crustal residence ages calculated for other sm nd data available from the churchill province the deep drill hole samples from the interior platform give sm nd crustal residence ages that average 2 8 ga granulites from northeastern alberta and composite gneisses from northern saskatchewan also provide archean crustal residence ages these data demonstrate that the presence of reworked archean crust in the western churchill province is more widespread than has been confirmed previously we suggest that sm nd crustal residence ages are valuable estimates of crust formation times especially in orogenic areas where other isotope systems have been disturbed

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

the provenance and crustal residence ages of british sediments in relation to palaeogeographic reconstructions 24 smnd isotope analyses of fine grained phanerozoic and modern clastic sediments from britain and quebec are presented in combination with published data they have been used to calculate crustal residence ages tcr and to assess the provenance of the british sedimentary mass sediments now preserved on either side of the suture formed by the closure of the iapetus ocean ca 400 ma ago were derived from isotopically distinct source regions sediments north of the suture are characterised by1 7 tcr 2 8 ga whereas those to the south exhibit a smaller range and an average value of 1 6 ga the northern and southern source regions were most probably laurentia and gondwana respectively it seems likely that a third source perhaps baltica provided the lower palaeozoic southern uplands succession sediments deposited in southern britain after the closure of iapetus were derived mainly from the recycling of older sediments the tectonic rearrangements which occurred during the phanerozoic are not reflected in the smnd isotopic structure of the southern british sedimentary mass suggesting that the caledonian and hercynian orogenies and even the grenville orogeny involved minimal accretion of new mantle derived material into the british and adjacent continental crust smnd analyses of fine grained clastic sediments provide a powerful sedimentological tool for elucidating palaeogeography clastic source areas sediment recycling and maturity and some aspects of sediment transport

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

decoupled evolution of nd and sr isotopes in the continental crust and the mantle nd and sr isotopes evolve independently in clastic sedimentary rocks and the continental crust as a whole because sr is recycled preferentially from the continents to the mantle as a result the differentiation age of the mantle inferred from sr isotopes and the mean rb sr age of the continental crust are younger than their true ages

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

the growth of the continent through geological time studied by nd isotope analysis of shales abstract we determined the nd isotopic composition and the sm nd ratios in a series of australian shales ranging from 0 2 gy to 3 3 gy the first result of this study is the constancy of the sm nd ratio in these shales as in granitoids secondly the initial 143nd 144nd ratio gives a regular curve decreasing through geological time both results confirm that shales are representative samples of the continental crust when insoluble elements like ree are studied we calculated their nd model ages of crustal differentiation the model ages regularly decrease with the stratigraphic ages and after 2 gy the curve flattens and tends to an asymptotic value at around 1 8 gy the significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin from these results we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas we propose that the results obtained here are representative of the whole gondwana continent having studied the case of the australian shales we try to extend our study first we applied our quantitative approach to the recent results obtained by o nions et al on north atlantic provinces the continental growth curves obtained by our inversion procedure are quite distinct from the australian shales showing the regional character of shales with these two cases studied we try to compare our results with the already developed model for continental growth the consideration of the surface conservative versus surface non conservative models clearly show the non uniqueness of the geological interpretation of the growth curve on the other hand we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the hurley rand province age curve such recycling increases with time in agreement with the data obtained on nd and sr initial ratios on granitoids

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic