mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

geochemistry and provenance of the middle ordovician austin glen member normanskill formation and the taconian orogeny in new england the austin glen member of the upper middle ordovician normanskill formation is a sandstone shale ysch succession deposited in the foreland of the taconian orogen petrographic major and trace element and nd pb isotopic data provide substantial constraints on its provenance lack of k feldspar and paucity of plagioclase in addition to the dominance of sedimentary rock fragments indicate that the source was dominated by recycled sedimentary components major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same no evidence of an ophiolitic or volcanic component was observed interpretation of nd isotopic characteristics are complicated by a partial resetting of the nd isotope system at about the time of sedimentation but indicate that the provenance of the austin glen member had a long term history of light rare earth element lree enrichment average tdm 1a8 ga furthermore nd isotopic compositions are extremely homogeneous end 8a1 0a6 1 s d n 23 at 450 ma the approximate depositional age indicating either a single source or very well mixed sources 207pb 204pb ratios are variable but within the range of pb isotopic compositions typically described as grenvillian the range of 207pb 204pb is greater than expected for the range of 206pb 204pb and suggests an additional component of pb possibly introduced during diagenesis the immediate source of the austin glen member may have been the accretionary prism that developed as older sediments of the laurentian margin were scraped off the basin oor incorporated within the accretionary prism and shed into the basin no evidence indicating the arrival of an undifferentiated island arc or continental fragment during the taconian orogeny has been found the data acquired during this study can be explained almost exclusively by grenville province source components but with possible additional contributions from older laurentian terranes and late proterozoic rift volcanics that are not readily quanti ed but likely to have been minor accordingly we conclude that the taconian orogeny in new england involved either 1 a continental arc that involved exclusively laurentia 2 collision of a continental block with identical geochemical characteristics as laurentia or 3 essentially no detritus from any exotic colliding block island arc or continental fragment reached the foreland basin at the time of austin glen deposition

age relations among oslo rift magmatic rocks implications for tectonic and magmatic modelling abstract this paper presents a compilation of 67 published and unpublished radiometric age determinations by the rb sr method on magmatic rocks in the continental oslo rift in southeast norway the data include most major rock types and structural units in the subaerial part of the rift the implications of these data with respect to the tectonomagmatic evolution of the rift and processes in the mantle and crust are discussed the oslo rift was active for about 60 million years the earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre rift lower paleozoic sediments at about 300 ma extrusion of plateau lavas of basaltic and intermediate composition rhomb porphyries started about 295 ma and lasted to about 275 ma with largest magma volumes and highest extrusion rate from 295 to 285 ma in the vestfold graben segment gs one flow per 250 000 years and 0 30 km3 per 103 years the main faulting activity and graben formation occurred during this period the plateau volcanism was succeeded by a stage of bimodal central volcanoes most of which later underwent caldera collapses ring dykes and central intrusions in these calderas give ages in the range 268 266 ma in the vestfold and 266 243 ma in the akershus gs large composite intrusive complexes were emplaced in the periods 27 268 and 273 241 ma in the vestfold and akershus gs respectively a rate of propagation of the rift from ssw towards the nne has been estimated to 1 2 cm y 1 on the basis of 87 sr 86 sr initial ratios and type of magmatic products we estimate magmas to have been emplaced into the crust only during the first 20 25 ma of the magmatic period some of the magmatism appears to be initiated by movement along precambrian fault zones which were reactivated during the oslo rifting event

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

rare earth element redistribution and its effects on the neodymium isotope system in the austin glen member of the normanskill formation new york usa abstract three groups of ree patterns are observed for the sandstones and shales of the middle ordovician austin glen member of the normanskill formation one sandstone sample is enriched in middle and light rare earth elements the majority of samples are light rare earth element lree enriched with negative eu anomalies and flat heavy rare earth elements hrees whereas a third group appears to have lost lrees and middle rare earth element mrees homogeneous neodymium isotopic compositions for all samples at about the time of deposition nd 470 ma 8 2 1 1 2sd indicate that the source of the austin glen member was well mixed and that the neodymium had a long term enriched history the differences in the shape and abundances of the ree patterns combined with the neodymium isotope characteristics of the samples lead us to the conclusion that samarium and neodymium were fractionated and that the neodymium isotopic system might have been disturbed at about the time of deposition 470 ma most samples appear to have been affected by this process the two most altered shales lost about half of their neodymium and their sm nd ratios are higher than the sm nd ratios of average shales these two shales give clearly erroneous mantle extraction ages but the average tdm of the provenance can still be deduced 1700 1800 ma from the least disturbed sandstones with normal upper crustal sm nd ratios accordingly this study demonstrates that the rees may be transported and neodymium isotopes may be reequilibrated under certain sedimentary conditions e g diagenesis however in spite of evidence for ree redistribution provenance information tdm original ree patterns may still be inferred

diagenesis of low mobility elements ti rees th and solid bitumen envelopes in permian kennedy group sandstone western australia the permian kennedy group carnarvon basin western australia is a shallow marine shelf deposit of sandstone and interlaminated siltstone the original sand was apparently arkosic but was altered by dissolution of feldspar and by other diagenetic processes into a kaolinitic sideritic silicified quartz arenite after lithification hydrocarbons moved through the sandstone leaving microscopic solid bitumen envelopes that had formed around radioactive detrital minerals when irradiation polymerized crosslinked and locally solidified the oil most radioactive cores are monazite a few are zircon and xenotime some monazite grains contain minute elongate randomly oriented perforations that almost certainly represent fission tracks and are filled with bitumen the diagenetic sequence can be divided into three phases phase 1 the earliest is an episode of complex diagenesis phase 2 covers the time when pores were filled with hydrocarbons and the solid bitumen envelopes formed but other diagenetic reactions were inhibited phase 3 begins with the entry of formation waters phase 1 authigenic minerals comprise anatase anhydrite brookite florencite thorite halite muscovite pyrite quartz possible rutile siderite cryptocrystalline silica sphalerite and xenotime rare thorite thsio 4 locally replaces monazite siderite crystals contain nine zones the main variations being in fe and mg and contain solution cavities that may have formed in phase 1 two minerals not commonly reported as authigenic are florencite ce la al phosphate mainly as minute pseudocubic euhedra replacing feldspar and mica and xenotime ypo 4 as outgrowths on detrital zircon grains phase 2 diagenesis includes formation of the solid bitumen envelopes and phase 3 includes authigenesis of quartz kaolinite and zoned calcite and perhaps solution of some florencite the solid bitumen envelopes surrounding the radioactive accessory minerals enclose rare minute authigenic halite and anhydrite crystals whose absence elsewhere suggests that the envelopes preserved them from solution during phase 3 titanium dioxide probably from ilmenite or titano magnetite or both partly replaces feldspar and mica some aspects of the diagenesis are particularly significant 1 solid bitumen envelopes not only indicate the former presence of hydrocarbons but also enable the complex diagenetic sequence to be divided into three well defined phases 2 the envelopes can trap and preserve minerals apparently unable to resist solution outside the envelopes i e minerals that would otherwise have disappeared 3 the zoned siderite calcite and florencite indicate changes in pore water composition in the first and third diagenetic phases 4 ti and some ree were mobile at least on the scale of a thin section during early diagenesis

early magmatic phase in the oslo rift and its related stress regime abstract the earliest magmatic rocks in the oslo graben consist of a complex of sills and dykes intermediate to felsic syenites termed maenaites and basic camptonites intruded into cambrian and lower ordovician pre rift sediments no volcanism appears to be associated with this intrusive phase the sill intrusions are regional in extent and can be found in all parts of the oslo graben except in the north samples from six maenaite sill occurrences dated by the rb sr method yielded late carboniferous ages ranging from 304 to 294 ma these ages place the sill intrusions in the same evolution phase as the proto rift asker group sediments interpreting the sill complex as a paleostress indicator and applying the tectonic model for the sill forming mechanism the deviatoric stress in the crust at the time of intrusion is found to be compressive at the level of the sills and shallower in contrast the succeeding evolution of the oslo graben was dominated by tensile deviatoric stress the timing of the incipient magmatic event in the oslo graben and its state of deviatoric compressive stress support the concept that the oslo rift developed initially in response to the build up foreland compressional stresses in conjunction with the hercynian orogeny there is no evidence for an early pre rift crustal doming stage

geochemistry of the arvonia formation chopawamsic terrane virginia implications for source area weathering and provenance the metasedimentary arvonia formation in the piedmont province of central virginia ranges primarily from slates to garnet schists which unconformably overly middle ordovician metavolcanic rocks of the chopawamsic arc terrane the formation has typically been interpreted as late ordovician based on poorly preserved fossils perhaps having formed as a successor basin following accretion of the chopawamsic arc to laurentia however alternative interpretations have been proposed including deposition outboard of laurentia on chopawamsic arc crust that is prior to accretion furthermore the origin of the chopawamsic arc as peri laurentian or peri gondwanan is an unresolved issue we report whole rock major and trace element compositions as well as nd isotopic compositions of the arvonia formation and use these results to evaluate further its origin samples collected along the length of the outcrop belt show similar major element compositions which are typical of shales chemical index of alteration values 68 89 corrected for k metasomatism indicate intermediate to extreme levels of source weathering a majority of the samples show uniform rare earth element ree patterns that are light ree enriched with negative eu anomalies a few samples display anomalous ree patterns indicative of some disturbance of the light ree at approximately the time of deposition a variety of trace element values or ratios indicate that arvonia sediments are similar to shale composites thus upper continental crust nd isotopic compositions for eight samples are similar with end 5 6 to 8 9 400 ma collectively these results are consistent with derivation of arvonia sediments from a single source or well mixed sources of typical upper crustal composition the isotopic data are compatible with a laurentian provenance however they are equally compatible with a strictly chopawamsic crustal source based on available nd isotopic data

baddeleyite and zircon u pb ages from the kaerven area kangerlussuaq implications for the timing of paleogene continental breakup in the north atlantic abstract we report u pb zircon and baddeleyite age results for two mafic units in the kangerlussuaq area which are part of the paleogene east greenland igneous province the kangerlussuaq macrodyke which represents a feeder to some flood basalts and the nearby kaerven gabbro baddeleyite from the macrodyke yielded a concordant age of 54 7 0 4 2 ma and this is among the most precise age determinations from the province this age is interpreted to reflect the time of emplacement the macrodyke belongs to a swarm of dykes emplaced perhaps in a failed arm during continental rifting and the age predates the completion of continental breakup at the end of emplacement of the plateau basalts of the east greenland eruption sequence geochemically the macrodyke is correlated with the milne land formation lavas the magma experienced slight crustal contamination based on the age and geochemistry of the macrodyke the age of the skaergaard intrusion and skraenterne formation lavas an age span of 1 2 ma is confirmed for the extrusion of the entire plateau basalts u pb analysis of zircon from the kaerven gabbro yields an age of 53 0 0 3 2 ma the kaerven gabbro was intruded after extension had ceased in the area and we suggest that the post plateau basalt magmatism did not take place at a distinct later time but instead that igneous activity continued for some time at a lower rate slightly fractionated hrees indicate that the kaerven gabbro magma was extracted at relatively shallow levels in the mantle

stratigraphic relationships and detrital composition of the medial ordovician flysch of western new england implications for the tectonic evolution of the taconic orogeny a reply to rodgers we present new stratigraphic and petrographic data on the easterly derived medial ordovician flysch sequence of western new england and eastern new york state a conformable relationship exists between the flysch and underlying early paleozoic continental rise sediments within the western taconic allochthon farther east in the allochthon we observe a possibly disconformable relationship contrary to previous interpretations these facts suggest that at any place within this paleogeographic realm the sediments were undeformed and untransported at the initiation of flysch deposition at least three sources contributed detritus to the flysch 1 taconic sequence sediments 2 deformed and metamorphosed pelitic psammitic and volcanic lithologies and 3 igneous rocks of dominantly mafic ultramafic affinities these three sources are respectively interpretable as continental rise accretionary prism and ophiolitic volcanic arc assemblages which are recognized in the regional geology of western new eng

a petrographic approach for evaluating trace element mobility in a black shale abstract at the time of deposition black shales record geochemical information related to the provenance and conditions of deposition for that rock these initial geochemical conditions are susceptible to alteration and overprinting by the aggressive processes of black shale diagenesis in this study we evaluate a sequence of ordovician black shales from the southern welsh basin for the presence of such a diagenetic overprint through use of cathodoluminescence backscattered electron imaging reflected light microscopy fission tracks and microprobe elemental mapping we can discern textural relationships with in the welsh black shales the relationships observed constrain a paragenetic sequence and overall diagenetic history including the early diagenetic formation of pyrite apatite rare earth element ree rich and carbonate and later formation of diagenetic monazite ree u rich and late stage carbonate the chemical reactions necessary to produce this mineral sequence require postdepositional redistribution of ree including changing cerium anomalies ce ce and uranium as a result ree u and their associated isotopic systems sm nd and u pb can reflect a diagenetic overprint rather than the source area and depositional conditions the paragenetic sequence described in this study records several postdepositional changes in the whole rock mineralogy among these changes are the formation of ree rich apatite and ree and u rich monazite the formation of these phases requires that ree and u are being redistributed on at least a mineral scale and that the initial depositional and provenance information recorded by the whole rock has been disturbed therefore the approach used in this study to carefully characterize the paragenetic sequence from the llandeilo caradoc black shales should be considered necessary in order to fully evaluate the usefulness of trace element proxies susceptible to diagenetic redistribution

the effects of weathering on rare earth element y and ba abundances in tertiary basalts from southeastern australia tholeiitic basalts showing anomalously high ba y and rare earth element ree concentrations occur within the tertiary newer volcanic province of victoria and these geochemical characteristics are related to incipient alteration accompanying weathering ree and y released during weathering of basalts are retained either in secondary phosphate or clay minerals and in rocks showing very high ba concentrations the ba is present as baso4 leaching experiments suggest that glassy or fine grained rocks are more likely to release ree y and ba while more extensively altered rocks are likely to hold these elements probably because they substitute in or are adsorbed on clay minerals or because secondary baso4 and ree phosphates are resistant to acid leaching samples taken sequentially across a weathering profile in a tholeiitic basalt flow from fresh to intensively altered rock illustrate that in the very earliest stages of alteration extreme variations in the abundances of ba y and ree can occur this is reflected in the broad patterns of trace element behaviour manifested by an extensive analysed sample suite from the victorian newer volcanic province where there is no correlation between anomalous enrichments in ba y or ree and the level of incipient alteration ba anomalies are not necessarily accompanied by abnormal y and ree behaviour and this is probably because the mobility of each trace element is a function of a the ionic radius and charge of the ionic species it forms in solution b local groundwater parameters such as eh ph groundwater flux and concentrations in the groundwater of species such as so42 and c the secondary mineral assemblages developed in a particular situation sr isotopic compositions are not strongly affected by incipient weathering and do not show significant changes until alteration has progressed to the point where plagioclase begins to break down the study illustrates that relatively slight alteration at the earliest stages of weathering can produce drastic changes in the concentrations of ree y and ba and petrographic assessment of the freshness of samples is not precise enough to eliminate prior to analysis cases where these effects may be manifested where weathering has caused changes in abundance patterns these may be recognised by careful comparisons of appropriate elemental ratios e g ba rb throughout a particular analysed sample suite

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

size and spatial structure of the soil and lacustrine charcoal pool across a boreal forest watershed abstract forest fires convert a proportion of the burning vegetation into charcoal that is stored in forest soils and lake sediments in this paper we use a geostatistical approach to present a detailed analysis of the size of the charcoal pool and its spatial variation in a boreal forest watershed including its lake sediment the amount of soil charcoal averaged 179 g m2 and ranged from 0 to 3600 g m2 in the watershed there was an extreme variation in the size of the charcoal pool over fine cm spatial scales for example the amount of charcoal in the soil could range from 34 to 1646 g m2 within a distance of 10 cm individually dated soil charcoal particles had radiocarbon ages that varied from 630 to 2930 cal yr bp the lake sediment began accumulating at 10 600 cal yr bp and charcoal accumulation has been practically continuous ever since then with the largest peak occurring at 6900 cal yr bp the lake sediment contained more charcoal 360 g m2 than the average for forest soil we interpret this as an indication of a relatively rapid degradation of charcoal in boreal forest soils

high grade contact metamorphism of calcareous rocks from the oslo rift southern norway shallow level plutons caused extensive contact metamorphism of lower paleozoic shale and carbonate sequences in the permian oslo rift a 500 m long and 100 m wide shale limestone xenolith embedded within monzonites belonging to the skrim plutonic complex experienced high grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks the peak metamorphic stage i assemblages in calcite saturated rocks include wollastonite melilites fassaitic pyroxenes phlogopite titanian grossular kalsilite nepheline perovskite cuspidine baghdadite pyrrhotite and occasional graphite mineral reactions involving detrital apatite produced a series of silicate apatites including the new mineral species ca sub 3 5 th u sub 1 5 si 3 o 12 oh this assemblage equilibrated at t 820 870 degrees c with a c rich internally buffered pore fluid 20 40 mol co 2 ch 4 during cooling the shale limestone xenolith experienced infiltration of c poor 2 fluids triggering the formation of retrograde stage ii mineral assemblages comprising monticellite tilleyite vesuvianite grandite garnets diopside and occasional hillebrandite rare potassium iron sulfides rasvumite and djerfisherite formed at the expense of primary pyrrhotite these assemblages probably formed near 700 degrees c formation of diffuse sodalite bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar the sodalite bearing stage iii assemblage formed by the infiltration of saline brines at a maximum temperature of 550 degrees c low temperature stage iv retrogression of the stage i iii assemblage produced scawtite giuseppettite hydrogrossulars phillipsite thomsonite and three hitherto undescribed mineral species

contrasts in composition and evolution of tertiary cfbs between west and east greenland and their relations to the establishment of the icelandic mantle plume

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

long lived lree mobility in the cratonic rift and foredeep to foreland sedimentary cover at the western margin of the karelia province abstract the western margin of the karelia province of the fennoscandian shield consists of archean crust covered by remnants of paleoproterozoic c 2 5 1 9 ga metasedimentary rocks the study area in the eastern part of the margin shows 2 3 2 1 ga cratonic sedimentation followed by 2 1 2 05 ga rift sedimentation and c 1 91 1 92 ga foredeep to foreland sedimentation related to continent arc continent collision extreme lree mobility loss up 95 of la is recorded by many samples from these units monazite bastnasite and in some cases allanite have been the prealteration lree host phases in the lree depleted samples monazite was altered to th silicates with variable loss of the lree the remaining part of which was incorporated in th poor monazite the depositional age of the sedimentary units was studied by dating la mc icpms detrital zircon grains and the age of lree mobility by dating monazite and xenotime and by whole rock sm nd modeling the maximum deposition age of the foredeep to foreland basin is 1 91 1 92 ga based on detrital zircon data and the minimum age of the main foliation in the study area is 1 87 1 86 ga based on the younger age limit of late to post tectonic xenotime and the ages of few probable metamorphic zircons coeval bastnasite with xenotime in a lree enriched sample suggests that this was also one important stage of lree mobility monazite crystallizations have occurred 1 76 1 78 ga ago while the rb sr whole rock data suggest that the closure of this isotope system took place 1732 21 ma ago the sm nd modeling gives ages at 0 4 1 0 ga for lree depletion the sm nd age of 412 27 ma can be considered as the age of the latest major lree depletion event we propose that alkali bearing oxidizing fluids started to form in the cratonic sequences due to diagenetic reactions maybe as early as 2 3 2 1 ga alkalinity of fluids was later increased by interaction with alkaline 2 05 ga source materials before and during the c 1 91 1 92 ga foredeep to foreland basin stage basin inversion and metamorphic crystallization occurred at 1 91 1 87 ga episodes of fluid migration at 1 78 1 73 ga and afterwards were focused along fracture zones and in microfracture networks the 0 4 ga stage is the most prominent of the

linking changes in tectonic style with magmatism in northern europe during the late carboniferous to latest permian abstract early carboniferous to permian magmatism associated with rifting within the northern foreland of the variscan orogen was widespread across europe during the long period of magmatic activity the regional tectonic setting changed across the region from early carboniferous extension and basin formation to a rifting wrenching style of deformation in the late carboniferous stephanian to early permian wrenching and faulting were accompanied by widespread voluminous and episodic magmatic extrusion intrusion and underplating this was followed by thermal relaxation and the development of the northern and southern permian basins in later permian times thermal relaxation was punctuated by a permo triassic phase of extension and graben formation 40ar 39ar ar step heating dating for mineral separates and whole rock samples of magmatic rocks from southern scandinavia oslo graben and south sweden and rugen north germany provides further radiometric evidence for three of the proposed periods of magmatic activity in the region latest carboniferous to earliest permian ages c 300 310 ma were obtained for volcanic rocks in the oslo graben and dolerite sills and dykes in south sweden and north germany this phase can be time correlated with magmatic activity that occurred throughout europe during large scale dextral wrenching that followed the variscan orogeny a second phase of alkaline intrusions is confined to the oslo graben and related to caldera collapse around c 275 ma the third permo triassic phase c 250 ma is considered to be related to a new tectonic cycle involving extension that triggered minor melting of enriched fertile mantle

the oslo rift a review the tectonomagmatic history of the oslo rift may be subdivided into 5 main periods stage 1 300 ma development of a shallow depression stage 2 300 295 ma roughly simultaneous onset of widespread basalt volcanism b1 and vertical movement along nnw sse to n s trending faults stage 3 295 275 ma main rifting period accompanied by volcanism dominated by rhomb porphyry lavas latites stage 4 275 240 ma change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions many of which went through caldera collapses intrusion of composite batholiths of intermediate to granitic compositions stage 5 240 ma dike intrusions may have continued into triassic time both the crust and total lithosphere under the rift are thinned relative to the precambrian basement on both sides a 12 km thick high density and high velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift shoulder petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle derived magmas in deep crustal magma chambers two mantle sources one nearly undepleted age corrected nd of about 1 and sr of 10 to 15 the other mildly depleted nd of about 4 sr of 10 contributed melts to the oslo rift magmatism the two sources are interpreted as different parts of a heterogeneous upper mantle which before the rifting event belonged to the subcontinental lithosphere on their way to the surface the mantle derived magmas were retained in deep crustal magma chambers where they underwent extensive fractional crystallization some also suffered moderate contamination anatectic melts and melts formed by mixing between mantle derived and anatectic melts gave rise to syenitic and granitic intrusions

interoceanic variation in the rare earth major and trace element depositional chemistry of chert perspectives gained from the dsdp and odp record rare earth element ree major and trace element abundances and relative fractionations in forty nodular cherts sampled by the deep sea drilling project dsdp and ocean drilling program odp indicate that the ree composition of chert records the interplay between terrigenous sources and scavenging from the local seawater major and non ree trace element ratios indicate that the aluminosilicate fraction within the chert is similar to nasc north american shale composite with average pacific chert including 7 nasc like particles indian chert 11 nasc atlantic chert 17 nasc and southern high latitude shl chert 53 nasc using la as a proxy for ree approximations of laex the amount of la in excess of that supplied by the detrital aluminosilicate fraction indicate that pacific chert contains the greatest laex 85 of latotal and shl chert the least 38 of latotal as shown by interelement associations this laex is most likely an adsorbed component onto aluminosilicate and phosphatic phases accordingly chert from the large pacific ocean where deposition occurs relatively removed from significant terrigenous input records a depositional ree signal dominated by adsorption of dissolved rees from seawater pacific chert cece 1 and lanybn 0 8 1 resulting from adsorption of local ce depleted seawater and preferential adsorption of lrees from seawater e g lanybn 0 4 which increases the lanybn ratio recorded in chert chert from the atlantic basin a moderately sized ocean basin lined by passive margins and with more terrigenous input than the pacific records a mix of adsorptive and terrigenous ree signals with moderately negative ce anomalies and lanybn ratios intermediate to those of the pacific and those of terrigenous input chert from the shl region is dominated by the large terrigenous input on the antarctic passive margin with inherited cece 1 and inherited lanybn values of 1 2 1 4 cece does not vary with age either throughout the entire data base or within a particular basin overall cece does not correlate with p2o5 concentrations even though phosphatic phases may be an important ree carrier this and previous studies of the large scale controlling parameters of sedimentary rees across ocean basins collectively indicate that ree indices of depositional regime e g cece lanybn laex are reproducible in a variety of sediment and rock lithologies ages and ocean basins and present a coherent tool for paleoceanographic and tectonic basin reconstructions

ree fractionation and ce anomalies in weathered karoo dolerite abstract analyses of samples from a weathering profile on karoo dolerite allow elements to be divided into three groups depending on their behaviour si k na mg ca sr ba and v are mobilized and removed from weathered products fe al ti zr hf zn cu sc co and ni are immobile ree y and to a lesser extent cr are mobile and redistributed within the profile without a net loss of these elements from the profile large positive ce anomalies are developed in oxidized weathered products by preferential leaching of the other ree s negative ce anomalies and ree enrichment is a feature of less altered dolerite

elemental distributions and neodymium isotopic compositions of silurian metasediments western maine usa redistribution of the rare earth elements abstract pelitic schists from the lower garnet to lower staurolite zones from the rangeley perry mountain and smalls falls formations of western maine were analyzed for major elements trace elements and neodymium isotopes these formations were derived from highlands created during the taconian orogeny deposited into a trough and metamorphosed during subsequent orogenic events most major and trace element abundances relative to al 2 o 3 were statistically identical between zones of the same formation as well as between formations although the average major element composition of these formations are the same there are systematic variations in some elements notably plots of sio 2 vs al 2 o 3 and k 2 o vs al 2 o 3 suggest that most of the variation could be produced by mixing of a fairly constant ratio of clay minerals and feldspar with varied amounts of quartz due to sorting in the sedimentary system different amounts of these minerals should not influence the shape of the ree patterns of the metapelites but higher amounts of quartz and feldspar may dilute the rees and most elemental abundances of the clay minerals and lead to lower elemental abundances the major difference between the samples within the perry mountain formation are different lree and mree abundances relative to al 2 o 3 which are not correlated to differences in major element or other trace element abundances relative to al 2 o 3 the samples in the perry mountain with higher lree and mree abundances have for example 42 3 8 3 ppm and those with low abundances have 5 6 3 6 ppm the samples with the high ree abundances of the perry mountain formation are similar in abundances and ree patterns to those of the rangeley and smalls falls formations typical of mudstones derived from granitoids another difference between the low and high ree abundance samples are the calculated t dm model ages the high ree abundance samples of the perry mountain formation show t dm similar to the samples of the rangeley formation with ages of about 1 7 1 8 ga the perry mountain samples with low ree abundances however give unrealistically old t dm s between 2 5 and 5 3 ga these unrealistically old t dm s are due to the relatively high sm nd ratios compared to crustal values which are characteristic of samples of the perry mountain formation with lower ree abundances we therefore suggest that these samples may be indicators for open system behavior of the neodymium isotopic system the timing of this disturbance of the neodymium isotope system is difficult to determine and cannot be tied to weathering or a definite postdepositional event the complexities of the data suggest more than one resetting event the most likely event that could have produced much of the movement of the lrees and mrees could have been due to small scale migration between anoxic hemipelagites and turbidite mudstones during diagenesis but some migration may have continued during metamorphism in order to reconcile the neodymium isotopic data

mixing relationships and the effects of secondary alteration in the wishstone and watchtower classes of husband hill gusev crater mars 1 the wishstone and watchtower class rocks on husband hill preserve evidence for a geochemical relationship consistent with two component mixing between a high al 2 o 3 tio 2 cao na 2 o p 2 o 5 end member and a second end member enriched in the elements mgo zn s br and cl the first end member appears to be reasonably well represented by rocks of the wishstone class while the second end member is consistent with a chemical component not represented by any lithology encountered by spirit the watchtower class appears to be an intermediate in the mixture the concentration of the redox sensitive elements fe and mn display no systematic variation between rock classes and the fe redox state of the watchtower class is elevated relative to the wishstone class combined with evidence for two component mixing these observations suggest that the mixture was generated by interaction between an initially wishstone like deposit and a fluid phase enriched in the elements mgo zn s br and cl the fluid phase may have been weakly acidic ph 4 to basic ph 9 resulting in oxidation and immobilization offe and mn a number of secondary rock surface alteration processes which obscure the mixing relationship are also identified these include 1 removal of cao and p 2 o 5 from rock surfaces by dissolution of ca phosphate minerals under acidic low water to rock ratio conditions 2 addition of so 3 to rock surfaces via aeolian deposition of high so 3 soil and or precipitation of sulfate salts from solution and 3 homogenization of rock surface chemistry by soil contamination

effect of diagenesis on trace element partitioning in shales abstract in order to investigate the partitioning of trace elements in mineral phases of black shales and diagenetic remobilization of trace elements within these phases we isolated major black shale components sulfide carbonate silicate and organic and analyzed the isolated fractions as well as corresponding whole rock for their trace element composition by drc icp ms this study focused on black shales of different grades of thermal maturity from the ordovician utica shale magnafacies of quebec usm immature tmax 20 50 c ro 140 c ro 1 analysis of the isolated rock fractions revealed that the carbonate fraction preserves a sea water like rare earth elements ree abundance pattern with enrichment of the heavy rare earth elements hree dy lu relative to light rare earth lree la sm the sulfide and organic fractions show a slight enrichment in the lree mass balance calculations showed that the organic fraction contains up to about 20 of total trace element content of the whole rock with the sulfide and carbonate fractions each containing less than 10 the partitioning and fractionation of trace elements between different phases may have important implications for paleoenvironmental reconstructions and provenance studies our data suggest that the whole rock signature may be less representative of source area or depositional environment but rather reflects the diagenetic history this is especially the case in organic rich sediments such as black shales where the organic fraction hosts up to 20 of the trace element content of the whole rock

elemental mobility in phosphatic shales during concretion growth and implications for provenance analysis shale normalized rare earth element ree patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales from the pennsylvanian age cyclothems in the u s midwestern states of oklahoma kansas nebraska iowa and ohio as well as the mississippian age fayetteville shale of arkansas shale normalized enrichments in middle ree mree developed as the phosphatic concretions lithified under reducing conditions in pore waters of organic rich muds depletions in mree sm ho in host shales correspond to the mree enrichments in some nearby concretions rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggest exposure to oxygenated waters positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggest extreme reduction during lithification of the phosphorite coating flat ree patterns in both host shales and phosphorite concretions probably reflect detrital influence phosphate diagenesis can affect the trace element chemistry of black shale enough to alter provenance signals overall enrichment in ree in phosphorite relative to host shale suggests movement of ree as does the mree enriched phosphorite near the mree depleted host shale diagenetically mobile ree can affect la th sc relationships used in provenance analysis the extent of the diagenetic modification of la th sc distribution is related to the amount of p2o5 present typical la th sc distribution in host shale results when p2o5 values are below 0 5 high la abundance substantially skews la th sc distribution when p2o5 is above 5 0 but when p2o5 is between 0 5 and 5 0 the extent of the effect is unclear oxides of aluminum and other major elements may be useful in recording detrital influence and proximity to the shoreline in phosphorite concretions most of the concretions nearest to shoreline sources of detrital input have both flat ree patterns and elevated values of al2o3 al2o3 fe2o3

physical characteristics and evolutionary trends of continental rifts rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension they are widespread and occur in a variety of tectonic settings and range up to 2 600 m y in age the object of this review is to highlight characteristic features of modern and ancient rifts to emphasize differences and similarities in order to help characterize evolutionary trends to identify physical conditions favorable for initiation as well as termination of rifting and to provide constraints for future modeling studies of rifting rifts are characterized on the basis of their structural geomorphic magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts and it is concluded that while simple models can give valuable insight into specific processes of rifting individual rifts can rarely if ever be characterized by well defined trends predicted by these models more data are required to clearly define evolutionary trends and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories

mineralogic controls on ree mobility during black shale diagenesis abstract the llandeilo caradoc black shales of southwest wales u k record a paragenetic sequence that requires redistribution of rare earth elements ree on a mineralogic scale during diagenesis the main evidence for this redistribution is formation and replacement of ree rich apatite and monazite and variations in whole rock ree patterns the processes that produce these mineralogic changes create a diagenetic overprint on the llandeilo caradoc black shales that obscures geochemical information related to both provenance and paleoenvironmental conditions of deposition periods of open system conditions existed during the diagenetic alteration of the llandeilo caradoc black shales which allow an evaluation of the extent and timing of the ree redistribution inferred from petrography the observed variations in la sm ratios in the whole rock ree patterns are strong evidence that ree were redistributed on a scale greater than an individual hand sample this change in the whole rock ree pattern altered the cerium anomaly by as much as 20 and produced a range in the sm nd ratio from 0 14 to 0 20 this change in the whole rock sm nd ratio indicates a disturbance in the sm nd isotopic system at about 460 ma that can be directly linked to the formation and dissolution of diagenetic apatite and monazite during diagenesis one effect of this disturbance is to alter the range in nd model age for the llandeilo caradoc black shales from 1 6 to 1 8 ga to about 1 4 to 2 25 ga thus complicating any interpretation of provenance the ability to link petrographic observations from a black shale to measurable whole rock geochemical changes provides us with one tool to evaluate the presence of a diagenetic overprint on trace elements commonly used in paleoenvironmental reconstruction

tectonic synthesis of the taconian orogeny in western new england a tectonic synthesis based on stratigraphic and structural analysis of western new england is proposed for the ordovician taconian orogeny it emphasizes arc continental collision in which ocean floor continental margin and ensialic rift rocks were imbricated westward in a repeatedly deformed accretionary wedge continued compression displaced segments of the north american sialic crust to the west and deformed the earlier emplaced slices of the taconic allochthons which were derived from the continental margin critical arguments for this synthesis are 1 the west to east stratigraphic relations among the basal rift clastic rocks of the dalton pinnacle and hoosac formations of late precambrian to early cambrian age 2 the stratigraphic and sedimentological similarities between the rocks of the lower taconic sequence and rocks in the pinney hollow and underhill slices to the east and north of the green mountain massif 3 the environmental similarities between the cambrian and lower ordovician section of the giddings brook slice and the age equivalent section in the st albans synclinorium 4 the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices groups 1 and 2 of the taconic allochthons 5 the presence of synmetamorphic fault related structures in the youngest and highest slices group 3 of the taconic allochthons 6 the recognition of extensive thrust zones in the pre silurian eugeoclinal sequence east of the middle proterozoic basement of the housatonic berkshire green mountain and lincoln massifs 7 the location of the taconic root zone within the pre silurian eugeoclinal sequence 8 the recognition of numerous faults in the serpentinite belt 9 the similarity between the rocks of the moretown formation and modern fore arc basin sequences 10 the recognition that the volcanic arc continental complexes of the ascot weedon and bronson hill have been displaced westward over the moretown and or hawley formations along such faults as the bristol and coburn hill thrusts 11 the allochthonous and internally imbricated nature of the north american basement in the berkshire massif 12 the proposition that the housatonic green mountain and lincoln massifs as well as the middle proterozoic cored domes of southeastern vermont are also thick sialic slices of north american basement 13 the recognition of medium high to high pressure metamorphic mineral assemblages in the pre silurian eugeoclinal rocks of vermont and 14 the recent synthesis of isotopic age data by sutter and others 1985 on the basis of an analysis of the foregoing arguments and relationships a chronological sequence of seven structural sections between albany new york and the bronson hill anticlinorium in central massachusetts is used to depict the evolution of the taconian orogeny retrodeformed distances are based on structural overlap and restoration of the taconic slices to their depositional setting along the ancient north american continental margin these easterly younging diverticulated slices formed as a result of horizontal compression rather than gravity sliding this palinspastic analysis implies the following 1 approximately 1 000 km of shortening has occurred during the emplacement of the taconic allochthons and the subsequent imbrication of north american basement as thick sialic slices approximately 330 km of this shortening is attributed to multiple cleavage generations 2 repeated movement along such major surfaces as the cameron9s line whitcomb summit belvidere mountain thrust zone has buried the taconic root zone we suggest that the northern extension of this root zone is exposed to the east of the lincoln massif in vermont where the underhill pinney hollow and hazens notch formations are exposed these formations here considered thrust slices disappear along the belvidere mountain whitcomb summit thrust zone as it is traced southward into western massachusetts and western connecticut 3 taconian metamorphic rocks particularly the older medium high pressure rocks in northern vermont have been transported westward on such reactivated surfaces as the belvidere mountain thrust 4 the anticlinorial form of the middle proterozoic basement in the green mountain and lincoln massifs may have resulted from fault bend folding on deep mantle involved thrusts that developed late in the taconian orogeny

u pb systematics in volcanic and plutonic rocks of the krokskogen area resolving a 40 million years long evolution in the oslo rift abstract the development of the oslo rift started in the latest carboniferous in the northern foreland of the variscan orogen the initial intensive and mainly mafic magmatism at around 300 ma was followed by a period of some 40 m y of mafic and felsic volcanic and plutonic activity the present study applied u pb in zircon on rocks of the krokskogen area northwest of oslo to resolve in greater detail the sequence of events in this evolution a major challenge however has been the absence or paucity of zircon and their low quality for dating in some of the important units especially the alkalic latitic rhomb porphyries the earliest rhomb porphyry flow rp1 yields an age of 299 7 0 4 ma it was thus coeval with the earliest basaltic and alkalic eruptions in the southern part of the oslo graben flows rp6 and rp7 yield overlapping ages of 285 5 0 8 and 284 9 0 5 ma and rp11 is 280 2 0 5 ma the baerum oppkuven and heggelia caldera volcanoes and the slottet kjelsasite pluton developed at 276 7 to 275 9 ma while the oyangen caldera is somewhat younger at 273 272 ma a kjelsasite stock has an age of 265 4 0 6 ma predating the 262 3 0 5 ma blindern rhomb porphyry like dyke which is among the youngest magmatic expressions in the rift the new data stress the importance of the magmatic burst at around 300 ma possibly plume related along the entire oslo rift and sharpens our understanding of the timing of the subsequent protracted sequence of magmatic events reflecting continuous heating and stress in the lithosphere likely broadly controlled by processes accompanying the closing of pangea

scale and timing of rare earth element redistribution in the taconian foreland of new england clastic sedimentary rocks deposited on eastern north america in response to the taconian orogeny commonly have sm nd isotope relationships indicating substantial isotope disturbance near or subsequent to the time of sedimentation that may be associated with severe depletion in light rare earth elements lree affected units normanskill formation austin glen and pawlet members frankfort formation and perry mountain formation are widely separated both geographically western new york to western maine and stratigraphically middle ordovician to silurian a model is proposed for the most likely explanation of the observed ree and sm nd isotope relationships involving a two stage process in the first stage ree are redistributed on a mineralogical scale dissolution precipitation on a sample scale often with the involvement of ree enriched trace phases such as apatite and monazite this stage typically takes place during diagenesis but may also take place later during metamorphism and or recent weathering and results in isotope re equilibration on a sample scale the second stage occurs when one or more of these phases is redissolved and ree are transported on large advective scales where lree enriched phases are involved this gives rise to lree depletion in whole rocks the timing of this second stage cannot be constrained from sm nd isotope data and may take place at any time subsequent to the isotope re equilibration such complex histories of ree redistribution may result in serious errors in estimating nd model ages but not in estimating the nd isotope composition at the age of sedimentation thus sm nd ratios even of unmetamorphosed sedimentary rocks have to be carefully evaluated before the calculation of depleted mantle model ages for the provenance

further constraints on the temporal evolution of the oslo rift from precise u pb zircon dating in the siljan skrim area abstract precise u pb ages have been obtained from zircon and baddeleyite in plutonic rocks from the siljan skrim area in the vestfold graben segment in the south western part of the permian oslo rift in addition common lead data from feldspars in the same rocks are presented the measured common pb compositions of the feldspars are used to correct for initial common pb in the zircons the common pb data indicate that u has been present in the feldspars since the time of crystallisation probably in a late magmatic volatile phase in fluid inclusions the siljan skrim area comprises two intrusive complexes the northern skrim mykle complex consists of two large composite intrusive bodies the skrim larvikite and the mykle ekerite the southern siljan hvarnes complex consists of a series of monzonitic to syenitic intrusions which are nested in a systematic way indicating decreasing emplacement age from east to west u pb zircon ages range from 281 2 0 6 ma 2 to 279 8 0 7 ma for three samples from the northern part of the siljan skrim area and from 278 5 0 8 ma to 277 3 0 8 ma for four samples from the southern part of the area the geochronological data suggest that large amounts of magma were emplaced within very short time spans i e less than 2 ma for each of the two complexes the high precision of the u pb age results allows discrimination between intrusive activity in the northern and southern parts of the siljan skrim area and suggests that emplacement of magmas was episodic

llanvirnian ord iron ooids in baltoscandia element mobility ree distribution patterns and origin of the ree abstract the chemical and mineralogical compositions of llanvimian ordovician iron and associated phosphate ooids from sweden estonia and russia have been examined the oolites are generally thin and lean mud supported and dominated by fe oxyhydroxide goethite limonite ooids one swedish oolite contains a mixture of chamosite goethite and hematite ooids and it is suggested that the two latter types were derived from chamosite ooids by early diagenetic alteration the ree distribution patterns were unaffected by these reactions all ooids in the area show a typical negative eu anomaly inherited from the source material major and minor element contents in limonite and chamosite ooids are similar whereas their ree patterns are different limonite ooids have a lree hree ratio from 1 to 2 4 and chamosite ooids from 3 to 5 the amounts of ree in the ooids are strongly dependent on the amount of phosphate

sr and nd isotopic evidence for punctuated clay diagenesis texas gulf coast we report rb sr sm nd and mineralogical data for leached whole rock 1200 m independent of the duration of burial the exchangeable sr in the clay has remained an open system and has undergone isotopic exchange since that time minimum fluid rock ratios of 0 2 to 2 are modeled from sr isotopic data consistent with rock dominated open system diagenesis leachates and residues of the 0 1 zm fraction below the smectite illite transition appear to have achieved nd isotopic equilibrium during clay diagenesis and show no signs of subsequent exchange the clay residues have low 147sm 144nd 0 085 indicating that authigenic illite has a lower sm nd ratio than its bulk rock protolith conversely the coexisting exchangeable ree inventory has elevated sm nd as reflected by leachate 14vsm 144nd as high as 0 188 this sm nd fractionation in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid rock interaction in older rocks

rare earth elements in saanich inlet british columbia a seasonally anoxic basin rare earth element and manganese distributions have been investigated in saanich inlet british columbia a seasonally anoxic basin redox cycling of ce and mn takes place across an oxic suboxic boundary approximately 20 m shallower than the o2h2s interface the remaining strictly trivalent ree undergo scavenging above this oxic suboxic boundary and regeneration from dissolving host phases below it these processes result in pronounced removal of ce from solution above the oxic suboxic boundary due to oxidative precipitation along with preferential uptake of the lighter trivalent ree during scavenging below the oxic suboxic boundary regeneration of particulate ree into solution leads to a less pronounced ce depletion and a less pronounced hree enrichment in the anoxic bottom waters dissolved ree concentrations in the sediment pore waters exhibit an approximately ten fold increase across the seawater sediment interface but then decrease with increasing depth indicating removal from solution during later diagenesis calculated diffusive fluxes of the ree and mn from the sediments into the overlying anoxic water column are insufficient to account for the accumulated trace metal enrichments observed in the water column we propose an annual redox cycle for saanich inlet in which flushing causes extensive oxidative precipitation and scavenging of trace elements from the water column each autumn the same material is rapidly regenerated from the underlying near surface sediments when the basin next turns anoxic

sm nd isochron age and provenance of the argillites of the gunflint iron formation in ontario canada abstract sm nd rb sr and k ar isotope analyses were made on argillaceous sediments from the gunflint iron formation in the lake superior region in order to provide new information on their sedimentation age and evolutionary history the investigations indicate that it might be possible to date the time of sedimentation by the sm nd method by studying specific material such as argillaceous whole rocks enriched in organic matter and clay fractions the spread in the sm nd ratios seems here to have been favoured by the incorporation of organic material in the argillites the sm nd isotope data yield an isochron age of 2 08 0 25 ga which seems to be a fair estimation of deposition age model calculations indicate that the main source supplying the animikie seawater with ree was located at the northern border of the basin the argillites and their clay minerals did not behave as closed systems with regard to the rb sr and k ar systems during further diagenetic evolution

sm nd and u pb zircon isotopic constraints on the provenance of sediments from the amadeus basin central australia evidence for ree fractionation abstract the amadeus basin of central australia is a late proterozoic to late palaeozoic ensialic depositional basin located between two proterozoic basement blocks of different nd crustal formation ages the older arunta block t dm nd 2 0 to 2 2 ga u pb zircon ages 1 5 1 9 ga to the north and the younger musgrave block t dm nd 1 7 to 1 9 ga u pb zircon ages 1 0 1 7 ga to the south initial nd isotopic compositions of the amadeus basin sediments generally fall into the region defined by the evolutionary trajectories of these two basement blocks indicating that the sediments are dominated by essentially two source components the older arunta block 147 sm 144 nd 0 114 and nd 0 20 4 and younger musgrave block 147 sm 144 nd 0 118 and nd 0 14 7 and or their equivalents stratigraphically higher sediments plot more closely to the evolutionary trajectory of the musgrave block this result together with the average provenance ages indicates the proportion of material from the musgrave block increases as the sediments become younger the sm nd isotopic data preclude a substantial involvement of archaean sources u pb zircon ion probe analyses of the late proterozoic heavitree quartzite and the late cambrian goyder formation are consistent with the sm nd isotopic constraints most of the zircons from the basal heavitree quartzite give concordant u pb zircon ages of 1500 to 1900 ma consistent with derivation from an arunta type source whilst zircons from the younger goyder sandstone give six discrete u pb zircon age groups of 511 20 615 15 960 109 1190 54 1633 24 and 1878 48 ma indicative of a dominantly musgrave type source with a minor contribution from the arunta block or reworked preexisting amadeus basin sediments the two youngest u pb zircon ages probably indicate some contribution from late proterozoic cambrian volcanics in central australia or a previously unrecognised younger local source in the basement blocks the 511 20 ma constrains the maximum depositional age of the sediment the amadeus basin sediments have a surprisingly wide range of 147sm 144nd ratios 0 077 0 136 irrespective of grain size and rock types in addition the calculated t dm nd values for samples from the same stratigraphic unit are well correlated with 147sm 144nd ratios with samples of different stratigraphic units forming sub parallel arrays these phenomena are interpreted in terms of ree fractionation and preferential sorting of preexisting ree rich phases during sedimentary recycling and deposition this process can lead to either an increase or a decrease in 147sm 144nd ratio of a sediment relative to its source and therefore an erroneous estimate of its t dm nd provenance age a theoretical model is developed which accounts for the observed correlation between t dm nd and 147sm 144nd ratios for suites of stratigraphically related sediments and allows a reliable estimate to be made of the provenance age of individual stratigraphic units assuming the average 147sm 144nd ratio of the source is known or can be estimated the provenance age for each stratigraphic unit of the amadeus basin has been calculated using this method the sm nd isotopic systematics in the amadeus basin sediments suggest that 1 1 ree fractionation during sedimentary recycling can in some cases be an important factor and needs to be considered 2 2 the provenance of the amadeus basin sediments was controlled by local source regions and cannot be used to infer large scale continental averages hence some caution must be evoked in using sm nd isotopic constraints from restricted sampling for more generalised models of crustal evolution

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

rare earth major and trace elements in chert from the franciscan complex and monterey group california assessing ree sources to fine grained marine sediments abstract rare earth element ree major and trace element analyses of 77 samples from the mesozoic franciscan complex and miocene monterey group of california usa indicate that magnitudes of the ce anomaly ce ce and total ree abundances sree in these rocks are controlled dominantly by a the amount of included metalliferous material b the amount of direct terrigenous input and c the overall burial rate the relative importance of these individual processes varies across an ocean basin in such a way that ree relative fractionations and sree correspond with its depositional environment our conclusions are supported by an extensive review of published ree data from various river coastal open ocean and ridge influenced waters as well as terrigenous pelagic and metalliferous sediment the chert sequences record deposition in spreading ridge open ocean basin and continental margin environments the relative importance of metalliferous and terrigenous influences and the role of scavenging from seawater may be estimated from a sample s inferred depositional distance km from the spreading ridge the major element chemistry and the stratigraphie context the ce ce values are low 0 29 in the vicinity of spreading ridges and trend to successively higher 1 values with decreasing metalliferous and increasing terrigenous influences other ree indicators ree lu n la n eu eu also respond to metalliferous and terrigenous influences near the spreading ridge chert ree appears determined by burial rate while shale ree responds to metalliferous abundance in open ocean and continental margin environments ree is controlled by sediment exposure time and therefore is related inversely to burial rate ree data are consistent with the major elements fe2o3 mno and al2o3 in reflecting the respective metalliferous and terrigenous sources p2o5 and cao do not vary with ce ce in any environment but cao shows a weak negative correlation with ree trace element trends e g v cr cu ge si rb sr y th u in general can similarly be accounted for in the context of depositional position relative to metalliferous and terrigenous inputs rees in chert appear less affected by post depositional processes than either major or trace elements indicating that rees are better tools for paleoceanographic or tectonic reconstructions that are based on chert chemistry this work complements recent efforts to elucidate the factors determining large scale ree distributions in marine sediments and has direct bearing on previous attempts to use the ce anomaly in older rocks and sediments as a recorder of secular variations and anoxic events in the overlying water column

rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments abstract analyses have been made of ree contents of a well characterized suite of deep sea 4000 m principally todorokite bearing ferromanganese nodules and associated sediments from the pacific ocean ree in nodules and their sediments are closely related nodules with the largest positive ce anomalies are found on sediments with the smallest negative ce anomalies in contrast nodules with the highest contents of other rare earths 3 ree are found on sediments with the lowest 3 ree contents and vice versa 143 nd 144 nd ratios in the nodules 0 51244 point to an original seawater source but an identical ratio for sediments in combination with the ree patterns suggests that diagenetic reactions may transfer elements into the nodules analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to ree contents of nodules and sediments is negligible inter element relationships and leaching tests suggest that ree contents are controlled by a p rich phase with a ree pattern similar to that for biogenous apatite and an fe rich phase with a pattern the mirror image of that for sea water it is proposed that 3 ree concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate processes which favour the enrichment of transition metals in equatorial pacific nodules favour the depletion of 3 ree in nodules and enrichment of 3 ree in associated sediments in contrast ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions

the development of middle rare earth element enrichments in freshwaters weathering of phosphate minerals abstract many phosphatic minerals of biogenic authigenic and diagenetic origin are distinguished by rare earth element ree compositions which have a well developed middle rare earth element mree enrichment defined by a gradual increase in the shale normalized ratios of ree toward the middle of the ree series la to lu also the ree composition of dissolved matter in the waters of the amazon fly and sepik rivers as well as other river lake and ground waters have well developed mree enrichments the development of mree enrichments was studied through a series of leaching experiments using river bank sediments from the amazon fly sepik and mississippi rivers a well characterized phosphate mineral rich shale with a pronounced mree enrichment in its bulk composition was also leached in order to better assess the relationship between the weathering of phosphate minerals and the formation of mree enriched solutions the two leaching solutions per sediment consisted of filtered lake water at ph 5 and the same water adjusted to ph 2 leaching experiments with the four river sediments result in extensive fractionation whereby the ree compositions of the solutions develop mree enrichments from starting sediments that show no mree enrichments the ph 2 leachates have much higher concentrations of the ree and more fully developed mree enrichments than do the ph 5 leachates the mree enrichments at ph 2 usually form within an hour of starting the experiments the phosphatic shale results show that there is preferential release of large amounts of mree at ph 5 as well as at ph 2 demonstrating the weather ability of the phosphatic minerals at phs of natural freshwaters the sepik river ph 5 experiment also shows significant mree enrichment under near neutral conditions the development and retention of the mree enrichments in the leaching solutions is accompanied by an increase in dissolved phosphate concentrations although the magnitudes of the mree enrichment do not directly correlate with the percent of phosphate released the experimental data suggest that the preferential dissolution of phosphatic minerals during weathering can result in extensive ree fractionation between bulk river sediment and water leading to freshwaters with mree enrichments

age and tectonic implications of paleoproterozoic granitoid intrusions within the nain province near nain labrador u pb zircon age determinations from three granitoid intrusions in the nain region formerly assigned to the well known 1 35 1 29 ga mesoproterozoic nain plutonic suite nps indicate that they

low temperature aqueous mobility of the rare earth elements during sandstone diagenesis abstract diagenetic francolite carbonate fluor apatite occurs as overgrowths on detrital apatite grains in sandstones from the lower jurassic statfjord formation of the north sea the francolite overgrowths are considerably more enriched in the rare earth elements up to 1 wt ce 2 o 3 sr and f than the detrital cores these elements were carried in aqueous solution probably in the form of complex ligands involving organically sourced carbon and halogens it is possible that reported aberrant neodymium isotope model ages within the statfjord formation are the result of mobilization and relative fractionation of sm and nd during diagenesis rather than a result of changes in provenance

the genesis of zr nb ree mineralisation at khalzan buregte western mongolia reconsidered abstract the sources and formation conditions of unconventional zr nb ree mineralisation ree rare earth elements presently found in increasing number worldwide are still poorly constrained one particular problem is the specific role of magmatic and hydrothermal processes active in various geological settings investigation of zr nb ree mineralisation at khalzan buregte and tsakhir western mongolia enables to evaluate magmatic processes preceding economic mineralisation and in a second step to compare similar ore forming processes developing in host rocks of contrasting rock composition low vs high silica rocks the genesis of the zr nb ree mineralisation is re assessed using field observations whole rock analysis chemical composition quantitative modal analysis by x ray diffraction and by the application of various transmitted light and electron microscopic techniques coarse grained intrusive bodies dikes and volcanic rocks of alkaline silica saturated composition were found to be contemporarily emplaced at subvolcanic to volcanic levels forming four alkaline massifs within the khalzan buregte area the whole rock composition of weakly altered magmatic rocks ranges from syenite to quartz monzonite and alkaline granite alkali feldspar syenite to alkali feldspar granite according to their modal composition magmatic and at least two subsequent hydrothermal processes contributed significantly to the formation of economic concentrations of high field strength elements hfse such as zr hf nb ta ree and y in the khalzan buregte deposit and in the nearby tsakhir prospect mixing of magma from at least three sources and the formation of potassium feldspar cumulates resulted in local enrichment of zr nb and light rare earth elements lree in the rocks up to sub economic levels there was no significant increase in y and heavy rare earth elements hree during magmatism multistage metasomatic alteration resulted in a pronounced chemical and mineralogical heterogeneity of associated alteration assemblages the main hosts of zr and hf in the ores are zircon and other zirconium silicates gittinsite catapleiite ca and elpidite the rare metals nb and ta are mainly contained in various types of pyrochlore khalzan buregte and to a lesser extent in fergusonite and other minerals tsakhir a large variety of ree and y bearing minerals have been identified including oxides fluorocarbonates and silicates early hydrothermal alteration by silica and carbonate rich fluids yielded extreme concentrations of zr nb and lree later alteration resulted in enrichment of y and hree in the latter case fluids were very rich in fluorine our preliminary genetic model assumes a carbonatite related fluid system responsible for the early alteration that occurred late during or postdating the intrusion extrusion of the silica saturated magmas a li f granite type fluid system was active during the late alteration the interplay of all these processes resulted in the formation of a complex economic zr nb ree mineralisation at khalzan buregte

upb pbpb and smnd dating of authigenic monazite implications for the diagenetic evolution of the welsh basin abstract authigenic monazite nodules from silurian turbiditic rocks in the central welsh basin are zoned with respect to nd isotope composition the cores are relatively radiogenic and the rim to core trend projects toward the composition of associated very early authigenic apatite concretions these authigenic phases therefore equilibrated with changing fluid composition as they formed the apatites and the monazite nodule cores may have included significant nd from organic matter or seawater whereas the rees in the outer parts of the monazite nodules were derived from clastic detritus the apparent sm nd age obtained from these phosphates is a mixing line which gives an ordovician age for these silurian rocks the u pb isotope system does not show similar trends and shows promise as an alternative method for dating such phosphates a date of 417 11 ma 2 mswd 107 agrees well with recent estimates of the time of the diagenetic smectite illite transition in the rocks of this study a pb pb isochron gave an age of 422 24 ma 2 mswd 0 9 this shows that the pb is in isotopic equilibrium and is undisturbed and that the elevated mswd of the u pb regression is caused by slight subsequent mobility in the u pb system the event dated by the monazite is probably compactional dewatering during accelerated mid silurian sediment loading in the central welsh basin

resetting of neodymium isotopes and redistribution of rees during sedimentary processes the early proterozoic chelmsford formation sudbury basin ontario canada abstract the rees abundances and neodymium isotope compositions of sandstones and siltstones from the early proterozoic chelmsford formation reveal that the rees in some samples were redistributed during the early proterozoic by surficial processes and that prior to redistribution the nd isotope system was partially reset the chelmsford formation is a coarse grained turbidite sequence the youngest of three clastic units that fill the sudbury basin in ontario it is constrained to have been derived from nearby late archean superior province rocks by petrography major and trace element chemistry and pb isotopic compositions of whole rocks and feldspar although ree patterns and nd model ages for samples from around the base of the formation reflect their derivation from undisturbed superior province rocks tdm 2800 ma five samples from higher in the stratigraphic section show evidence that up to 60 of their la has been removed and sm nd has increased by up to 60 these samples also have anomalously high nd model ages tdm 3400 to 4400 ma the presence of pronounced positive ce anomalies in the samples with anomalous tdm indicates the influence of surficial processes on the ree composition the data scatter about a vertical array on a plot of f sm nd vs ndt 1800 nd calculated at 1800 ma approximately the depositional age with values of f sm nd between 0 45 and 0 14 mixing of nd from other sources is difficult to reconcile with the petrography major and trace element geochemistry and nd and pb isotopes removal of an lree enriched phase from the sedimentary system provides a viable mechanism to produce the ree and nd isotopic characteristics of the chelmsford formation samples neodymium isotopes require that this phase was partially isotopically equilibrated with the matrix sediment near the time of sedimentation this study shows the dangers of indiscriminate use of nd model ages in provenance studies and demonstrates that the simultaneous examination of sm nd and nd isotopes on f sm nd vs nd or isochron plots is required for the recognition of sedimentary redistribution of the ree

temporal changes in nd isotopic composition of sedimentary rocks in the sevier and taconic foreland basins increasing influence of juvenile sources initial 143 nd 144 nd ratios of clastic sedimentary rocks in the ordovician taconic and sevier foreland basins increase with decreasing stratigraphic age this shift in isotopic composition reflects an increased influx of juvenile sediment to both basins for the taconic basin geologic and isotopic data are consistent with the juvenile sediment being derivedfromupliftedgrenvillecrust upthrustaspartofthetaconicallochthons sediment derivedfromthebronsonhillarc oracombinationofboth forthesevierforelandbasin there are few geologic constraints on source regions but the similar shift in isotopic ratios with time suggests a similar increase in the influence of juvenile sources such as exotic terranes sevier basin sedimentary rocks have higher nd values than those within the taconic basin thus juvenile source areas were of greater importance to sevier basin sediments