oligocene miocene burial and exhumation of the southernmost gangdese mountains from sedimentary and thermochronological evidence abstract the kailas conglomerates crop out ubiquitously along the southernmost boundary of the gangdese batholith they unconformably overlie the gangdese batholith and are displaced by the great counter thrust gct fault forming a fault contact with the xigaze forearc basin the associated subduction complex and the tethyan himalayan sequence these strata furnish a record of uplift and paleoenvironmental change in the indus yarlung suture zone during the oligocene miocene our new and previously published low temperature thermochronometric data from the gangdese batholith and the kailas conglomerates indicate a period of rapid exhumation beginning approximately 17 15 ma centered on the southern margin of the gangdese batholith whereas regional uplift commenced significantly earlier during the deposition of the kailas conglomerates based on the presence of an abrupt facies transition from deep water lacustrine deposits to red alluvial fan or fluvial deposits the period of rapid exhumation probably lagged behind the initiation of faster uplift while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy subsequently the kailas conglomerates were buried in association with the development of the north directed great counter thrust while rapid exhumation was facilitated by efficient incision by the paleo yarlung river at approximately 17 15 ma

the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

coesite eclogites from the roberts victor kimberlite south africa abstract coesite or pseudomorphic quartz is more common than previously recognized in the eclogite xenolith suite from the roberts victor mine south africa all coesite eclogites in this study 18 samples are classified as group i and have mineral compositions that span virtually the entire compositional range of group i eclogites described from this locality most of the roberts victor group i eclogite suite may be in equilibrium with free silica thus weakening the case for these rocks representing residues of partial melting oxygen isotope compositions of garnets from 15 samples are in the range 5 32 6 95 similar to other roberts victor group i eclogites these observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures to variable extents prior to subduction and metamorphism to eclogite facies without suffering significant partial melting during or following subduction

redundant 230th 234u 238u 231pa 235u and 14c dating of fossil corals for accurate radiocarbon age calibration abstract 230th 234u 238u dating of fossil corals by mass spectrometry is remarkably precise but some samples exposed to freshwater over thousands of years may gain and or lose uranium and or thorium and consequently yield inaccurate ages although a 234uinitial value equivalent to modern seawater and modern corals has been an effective quality control criterion for samples exposed to freshwater but having 234uinitial values indistinguishable from modern seawater and modern corals there remains a need for additional age validation in the most demanding applications such as the 14c calibration fairbanks et al 2005 radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230th 234u 238u and 14c dates on pristine corals quaternary science reviews 24 16 17 1781 1796 in this paper we enhance screening criteria for fossil corals older than 30 000 years bp in the fairbanks0805 radiocarbon calibration data set fairbanks et al 2005 by measuring redundant 230th 234u 238u and 231pa 235u dates via multi collector magnetic sector inductively coupled plasma mass spectrometry mc ms icpms using techniques described in mortlock et al 2005 230th 234u 238u and 231pa 235u ages from a single fossil coral fragment by multi collector magnetic sector inductively coupled plasma mass spectrometry geochimica et cosmochimica acta 69 3 649 657 in our present study we regard paired 231pa 235u and 230th 234u 238u ages concordant when the 231pa 235u age 2 overlaps with the associated 230th 234u 238u age 2 out of a representative set of 11 fairbanks0805 fairbanks et al 2005 radiocarbon calibration coral samples re measured in this study nine passed this rigorous check on the accuracy of their 230th 234u 238u ages the concordancy observed between 230th 234u 238u and 231pa 235u dates provides convincing evidence to support closed system behavior of these fossil corals and validation of their 230th 234u 238u dates for radiocarbon calibration and 14c reconstruction

an ion microprobe study on trace element composition of clinopyroxenes from blueschist and eclogitized feti gabbros ligurian alps northwestern italy some petrologic considerations abstract detailed microtextural investigations coupled with ion microprobe trace element determinations have been performed on clinopyroxenes from blueschist and eclogitized fe ti gabbros from ligurian alps northwestern italy ion microprobe analyses have also been carried out on garnets from eclogitized fe ti gabbros the aim is to understand the relations between metamorphic reactions and trace element compositions of clinopyroxenes in high pressure low temperature terranes trace and major element analyses on relicts of igneous diopsides indicate similar protoliths for both eclogites and blueschists metamorphic clinopyroxenes have variable trace element compositions which reflect the compositions of original mineral phases in particular the clinopyroxenes replacing the igneous diopside commonly retain geochemical signatures of the precursor mineral such as relatively high sc y and hree contents the trace element variations among high pressure clinopyroxenes in different textural domains are greater for blueschists than for eclogites this reflects the more limited major element equilibration of blueschists relative to eclogites in deformed eclogites the clinopyroxenes lack the geochemical signatures of the original igneous minerals most probably due to the enlargement of reaction domains caused by synmetamorphic deformation trace element partitioning between omphacite and garnet suggests that in both undeformed and deformed eclogites chemical equilibrium is locally approached and therefore the trace element composition of clinopyroxenes is controlled by the coexisting minerals for instance clinopyroxenes in eclogites are strongly hree and lree depleted i e with bell shaped pattern due to the equilibration with garnet and allanite respectively the partition coefficients for rees between omphacite and garnet are markedly lower than those reported in the literature on the basis of crystal chemical considerations we suggest that this is probably due to lower temperature conditions during formation

paleogene island arc collision related conglomerates yarlung tsangpo suture zone tibet abstract coarse clastic rocks in the liuqu conglomerate formed in both terrestrial and subaqueous settings record a paleogene phase in the tectonic evolution of tibet facies changes are commonly abrupt with rapid changes in clast types grain size and stratal patterns sediments were derived from the leading northern edge of the indian margin and a late jurassic cretaceous intraoceanic island arc that lay within tethys the coarse clastic sedimentary rocks of the liuqu conglomerates are extremely proximal but are locally offset relative to their original source terranes they record aspects of the history of collision between these terranes and are interpreted to have been deposited in oblique slip basins that developed along the zone of collision the absence of clasts derived from terranes to the north of the yarlung tsangpo suture suggests that basins associated with deposition of the liuqu conglomerate developed prior to the final collision between india and asia

palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks a case study from the palaeozoic ballantrae ophiolite scotland abstract palaeomagnetic studies of the ballantrae ophiolite southwest scotland are assessed in the light of palaeomagnetic and rock magnetic reliability criteria the remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction rock magnetic reliability criteria are found to be ambiguous when treated in isolation original grain size effects are detected in magnetic properties across three sections through pillow lavas we propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability thermomagnetic analyses and microscopic observations suggest three remanence carrying magnetic phases exist within the extrusive layer of the ophiolite these are almost pure magnetite hematite and titanomaghemite palaeomagnetic rock magnetic and geological observations are best satisfied by a pre obduction magnetisation age this study links the remagnetisation of an ultra oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested

peraluminous xenoliths in kimberlite metamorphosed restites produced by partial melting of pelites abstract three new varieties of aluminous assemblages from the jagersfontein kimberlite are recognized as xenoliths consisting of garnet spinel alkremite corundum garnet corganite and corundum garnet spinel corgaspinite accessory minerals include phlogopite k pargasite fassaitic pyroxene and secondary spinel bulk rock major elements are depleted in sio2 and enriched in al2o3 and mgo relative to basalt the compatible trace elements ni cr zn ga and v reflect the modal mineralogy and the incompatible trace elements k rb and ba vary as a function of metasomatism chondrite normalized ree patterns are characterized by fanning light and heavy ree variations calculations show that the patterns are consistent with partial melting of pelitic rocks from which a granitic low temperature portion has been removed by anatexis to produce a restite containing the observed geochemical variations sr isotopic ratios of garnet separates are high 0 780 0 804 and also indicate a sedimentary origin for the assemblages a two stage process involving partial melting of archean sediments followed by prograde metamorphism of the residual restite is tentatively proposed initiated by exotic terrain accretion or subduction at the edge of the kaapvaal craton

petrology of the mafic rocks of the xigaze ophiolite tibet the xigaze ophiolite yarlung zangbo suture zone southern tibet china shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros by dolerites intrusive throughout the whole ophiolite sequence and by the injection of dolerites in already serpentinized peridotites the abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge all the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea water circulation for the spreading centre in which they have been formed in good agreement with a slow spreading ridge origin

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

vegetation and climate of the central and northern qinghai xizang plateau from the middle jurassic to the end of the paleogene inferred from palynology abstract the collision between india and eurasia and the subsequent uplift of the qinghai xizang plateau region of the eurasian continent during the latest mesozoic and cenozoic had a significant environmental impact on the region understanding the vegetation of both palaeocontinents prior to the collision is crucial for interpreting the evolution of their floras and climatic changes during this period the composition of the vegetation and climate of the plateau from the middle jurassic until the end of the oligocene are inferred herein from the composition of 14 palynofloras and compared with other regions of china the palynofloras are similar to those of eastern and northwestern parts of china but bear their own unique features brought about by differing climatic influences overall a warm hot arid semi arid climate prevailed over the plateau during most of the period from the middle jurassic until the end of the cretaceous the climate cooled and humidity increased in the paleogene and especially during the oligocene the long term climatic change in the qinghai xizang plateau region of the eurasian continent during the middle jurassic paleogene that is indicated by the miospore assemblages is broadly comparable to the isotope based sst sea surface temperature changes that have been identified it is apparent that climatic changes on a global scale were primarily responsible for controlling the composition and distribution of the vegetation on the plateau no monsoon features are detectable in the northwestern interior of china a maximum estimation of 2890 m surface palaeoelevation is inferred for the central and northern plateau during the oligocene based on vegetation analysis which is lower than previous estimations based on oxygen isotope and coexistence analyses the elevation of the southern part of the study area i e the area south of the gandise mountains was low close to sea level

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

last glacial sea level change deduced from uplifted coral terraces of huon peninsula papua new guinea we have dated corals sampled from the uplifted terraces of huon peninsula papua new guinea that grew during the marine isotope stage 3 to reconstruct sea levels of the last glacial period from 30 to 55 ka kiloannum mass spectrometric u series dating methods resulted in high precision dates and could also be used to recognize diagenetically altered samples the only quantitative test of open systembehaviour of both uraniumand thoriumisotopes involves the com parison of 234 u 238 u ratios at the time of coral growth with present sea water values additional tests included checks for systematic variations in stable isotopes x ray diffraction to detect calcification and petrographic examination sea levels were corrected for glacio hydro isostatic variations and also for the depth range of coral growth habitat sea levels from30 to 55 ka were approxim ately 80 mlower than present superimposed on this general trend four periodic sea level high stands were found at 6 8 ka intervals approximately at 30 38 44 and 52 ka that appear to be synchronous with rapid climate variations previously observed in ice cores and in deep sea sediments r 2001 elsevier science ltd and inqua all rights reserved

multi stage metasomatism of diamondiferous eclogite xenoliths from the udachnaya kimberlite pipe yakutia siberia the primary garnet pyrope almandine omphacite cpx 1 6 5 7 wt na2o sulfide fe ni co mss assemblage of the two diamondiferous eclogite xenoliths studied u33 1 and ux 1 experienced two mantle metasomatic events the metasomatic event i is recorded by the formation of platy phlogopite 10 wt k2o prior to incorporation of the xenoliths in the kimberlite the bulk of the metasomatic alteration consisting of spongy textured clinopyroxene cpx 2a 1 3 wt na2o coarser grained clinopyroxene cpx 2b 2 5 wt na2o pargasitic amphibole 0 8 wt k2o 3 3 5 wt na2o kelyphite cpx 3 mostly 1 wt na2o and zoned mg fe al spinel sodalite calcite k feldspar djerfisherite k5 95na0 02fe18 72ni2 36co0 01cu4 08s26cl and a small amount of k ca fe mg glass is ascribed to the metasomatic event ii that occurred also in the upper mantle but after the xenoliths were incorporated in the kimberlite a pervasive chloritic alteration mainly clinochlore magnetite that overprints earlier assemblages probably took place in the upper crustal environment the diamonds are invariably associated with secondary clinopyroxene and chlorite but the diamonds formed before the entrainment of the xenoliths in the udachnaya kimberlite

palynological stratigraphy of the late cretaceous and cenozoic collision related conglomerates at qiabulin xigaze xizang tibet and its bearing on palaeoenvironmental development abstract the stratigraphy age depositional environment and correlation of the collision related variegated sandstone and conglomerate beds exposed along the southern side of the gangdise mountains to the north of the yarlung zangbo suture in the xigaze region xizang tibet have been much debated generally referred to as the dagzhuka formation these beds of great thickness have until recently not been reported to contain any convincing fossils two distinctly different palynological assemblages indicating late cretaceous and oligocene early miocene ages respectively have now been recovered from lithologically distinguishable units of the sedimentary succession at qiabulin to the west of the town of xigaze this means that instead of one stratigraphic unit as has been generally accepted hitherto at least two are represented as a result the dagzhuga formation is considered here to refer only to the cenozoic deposits and the qiabulin formation previously regarded as a synonym is resurrected to accommodate the cretaceous beds these two formations were juxtaposed by tectonic movements our finding indicates that the stratigraphic successions on the southern laurasian continental margin of xizang reflect changes from a deep forearc basin through shallow littoral environments to a mountainous landscape during the late cretaceous and early cenozoic this interpretation compares well with those documented from the gamba and tingri districts to the south of the yarlung zangbo suture

latest marine horizon north of qomolangma mt everest implications for closure of tethys seaway and collision tectonics the newly discovered marine horizon at zhepure syncline north of qomolangma revises timing of the closure of the eastern neo tethys seaway in the central himalayas the marine pengqu formation conformably overlies a lutetian shallow water carbonate platform and comprises shales interbedded with sandstones deposited in a neritic shelf environment the strata are dated by nannofossils and foraminifera as late early lutetian to late priabonian age np15 np20 deposited 47 34 ma indicating that the final closure of the tethys seaway in this region occurred at 34 ma the newly discovered strata provide evidence about emplacement of a major thrust sheet south of southern tibet prior to 37 ma which affected the regional climate

extending the radiocarbon calibration beyond 26 000 years before present using fossil corals fossil coral is an excellent archive to extend the radiocarbon 14c calibration beyond tree ring records however published coral data older than 26 000 years before present are too disparate for practical calibration we propose an explanation for these discrepancies trace amounts of secondary calcite and organic matter in fossil corals shift the 14c ages toward younger ages in a series of acid leaching experiments to simulate the standard cleaning procedure prior to 14c analysis we measure up to a 300 enrichment of diagenetic calcite in corals due to the solubility differences between calcite and aragonite in model calculations we show the 14c age offsets produced by acid leaching could be hundreds to thousands of years when typical amounts of calcite are present in samples we demonstrate the necessity and our ability to detect 0 2 calcite in aragonite by x ray diffraction and apply 0 2 calcite as our a priori criterion for our coral samples the estimated age offsets of samples with 0 2 calcite fall within the reported analytical uncertainty of 14c dating in addition we present 14c results from an extended hydrogen peroxide h2o2 pretreatment for coral samples to remove organic matter our data show that treated samples yield consistent or older 14c dates compared to non treated samples these rigorous but necessary screening and cleaning techniques provide precise reproducible and accurate radiocarbon calibration data

eclogites with oceanic crustal and mantle signatures from the bellsbank kimberlite south africa part 2 sr nd and o isotope geochemistry the sr nd and o isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the bellsbank kimberlite erupted at 120 ma define three groups group a eclogites considered to be mantle cumulates are characterized by 6180 and 87sr srsr values typical of mantle derived materials 5 1 to 5 6 o and 0 7042 0 7046 respectively and very low sm nd ratios apparent rb sr ratios and nallz01 values 0 057 0 078 0 00005 0 00136 and 14 to 16 respectively the ree and isotopic data for these eclogites can be modelled in terms of crystallization from a group ii kimberlite magma at 1 1 5 ga group b and c eclogites believed to be the metamorphosed products of ancient subducted oceanic crust are characterized by low 3180 2 9 to 4 7 extremely high nd a20 40 to 219 and radiogenic 87sr s6sr ratios 0 708 0 710 the sm nd ratios of the group b eclogites are very high up to 1 6 the data for group b and c eclogites define a linear correlation on sm nd and 1 nd vs nal 201 diagrams these relationships are consistent with mixing of the bellsbank kimberlite cno 1201 10 sm nd 0 10 with a depleted eclogite end member end 120 219 sm nd 1 6 during a cryptic metasomatic event the sr isotopic variations in group b and c eclogites cannot be generated by simple two component mixing the st nd and o isotope data for group b and c eclogites probably reflect a complex sequence of depletion and enrichment events in both crust and mantle settings enrichments which possibly affected the group b and c eclogites include seawater alteration of a morb like protolith which lowered the 6180 and raised the svsr 86sr ratio but left the nd isotopic compositions unchanged and cryptic metasomatism by the magmatism that produced the bellsbank kimberlite the high sm nd ratio of the depleted eclogite end member cannot be generated by extraction of a melt from a modern morb composition rather it is argued that such high sm nd ratios are produced as a result of partitioning during the recrystallization of a morb like component to eclogite during subduction nd model ages suggest that this process occurred 2 3 2 4 ga

interlayered eclogites blueschists and epidote amphibolites from ne oman a record of protolith compositional control and limited fluid infiltration abbreviations ab albite ac acmite act actinolite alm aleclogites blueschists clinopyroxene rich rocks and epidote ammandine aln allanite ap apatite aug augite bar barroisite bt biotite phibolites occur interlayered on a millimetre to decimetre scale in a cc calcite chl chlorite cpx clinopyroxene crs crossite czo clinozoisite mafic unit interbedded with calcareous and quartz mica schists in dol dolomite ep epidote fe ox fe oxides gln glaucophane gross ne oman these rocks constitute part of the continental margin grossularite gt garnet hm haematite jd jadeite kln kaolinite kspar of oman subducted during the cretaceous petrological data indicate k feldspar mgt magnetite oxychl oxychlorite pg paragonite ph that they evolved through a clockwise p t path and attained peak phengite qz quartz pyrp pyrope rt rutile spess spessartine tc talc conditions at t 560 c p 12 kbar where the mineral ttn titanite win winchite zrn zircon assemblages of the blueschists eclogites and clinopyroxene rich rocks were all stable whole rock major trace and rare earth element data indicate that the mineralogical differences between these rock types resulted from the metamorphism of interlayered basaltic flows introduction and tuffs of variable compositions possibly reflecting different degrees interlayered blueschists eclogites and or greenschists are of magmatic differentiation the interlayered epidote amphibolites common in several areas such as cazadero california are chemically similar to the blueschists and appear to have formed e g oh et al 1991 the shuksan suite washington from the eclogites and blueschists during unroofing aided by fluid state dungan et al 1983 owen 1989 the tauern infiltration oxygen isotope data for minerals and whole rocks window eastern alps e g selverstone et al 1992 and indicate low fluid rock ratios during metamorphism and unroofing several cycladic islands e g okrusch brocker 1990 the data also suggest that different layers may have interacted with this interlayering has been attributed to 1 differences fluids of different isotopic signatures throughout their history these in the p t conditions of equilibration in the different results are similar to data reported for other areas of continent layers e g reinsch 1979 ridley 1984 2 original continent collision or continental crust subduction differences in the chemical compositions of the layers before metamorphism e g dungan et al 1983 oh et al 1991 or 3 differences in the extent of retrograde overprinting as a result of variable fluid infiltration e g schliestedt matthews 1987 brocker 1990 barrientos selverstone 1993

diagenesis and its impact on sr ca ratio in holocene acropora corals the effect of early diagenesis on sr ca ratios encapsulated in coral skeletons was evaluated by comparing mineralogical structural and geochemical characteristics of modern and holocene branching acropora colonies the modern specimens acropora danai acropora formosa come from reunion island western indian ocean and the great barrier reef of australia respectively the sr ca ratios of modern specimens range from 9 08 to 9 37 mmol mol the fossil acroporids acropora group danai robusta were collected from a 50 m core drilled through a barrier reef in tahiti island their c 14 ages range from 3 200 to 10 200 calendar years b p fossil skeletons are 100 aragonite earlier diagenesis has occurred in the marine environment it is expressed by growth of secondary inorganic aragonite over primary skeletal aragonite needles development of syntaxial aragonite cements within intraskeletal cavities and decrease in size of original 1 1 050 m wide pores residual porosity ranges from 25 to 28 which results in a volume reduction by 34 to 49 cementation increases with increasing age of the corals later diagenesis has occurred in a mixed marine freshwater environment it includes partial dissolution of skeletal and growth of cement aragonite fibres in the form of spherolites irregular meshes of large squarely terminated laths this results in an increase in porosity from 30 to 59 by reference to modern well preserved acroporids this diagenetic alteration has led to an increase of sr ca values from 9 08 9 37 to 8 89 10 55 mmol mol this variation in sr ca ratio can be linked to the increase in the amount of sr enriched cements relative to the volume of the skeletal aragonite and to a more homogeneous distribution of these cements throughout the skeleton the uncritical use of sr ca ratios as paleothermometers from diagenetically altered skeletons may cause serious misinterpretations accordingly estimate of the degree of diagenetic alteration in skeletons is a prerequisite to any paleoclimatic reconstruction based on coral records

oxygen isotope systematics of eclogite mineral phases from south africa abstract eleven eclogite xenoliths from the bobbejaan and roberts victor mines south africa have been characterized by a variety of geochemical techniques the samples include nine bimineralic eclogites and two grospydites that span the range of mantle eclogite major element compositions primary phase end member compositions range from py 75 al 16 gr 09 jd 18 di 82 caes 0 to py 28 al 21 gr 51 jd 49 di 36 caes 15 oxygen isotope analyses of mineral separates were performed for clinopyroxene and garnet isotopic values 18 o range from 4 0 to 7 8 for clinopyroxene and 3 6 to 7 2 for garnet differences in isotopic values between the primary phases clinopyroxene minus garnet range in value from 0 6 to 0 6 although oxygen isotopes are not expected to fractionate at mantle temperatures the anion electrostatic site potential values determined by smyth 1989 are consistent with small degrees of fractionation in the mantle oxygen isotope fractionation between minerals may be calculated by the equation of smyth and clayton f a 10 6 t 2 where a 0 7911 ev fractionation values at 1300 k range from 0 6 jd pyr to 0 03 cats grs while at 1600 k these values are between 0 39 and 0 02 respectively the re equilibration temperatures of the studied eclogite suite are 1050 to 1250 c at 30 kbar therefore the obtained differences between the mineral phases are consistent with the calculated isotope fractionation values the data obtained for eclogite oxygen isotopes suggest that 18 o may fractionate to a small extent at mantle pt conditions

archean mantle heterogeneity and the origin of diamondiferous eclogites siberia evidence from stable isotopes and hydroxyl in garnet data are presented for the o isotopic composition of clinopyroxene and garnet the c isotopic composition of diamond and the oh content of garnet from eclogite xenoliths brought to the surface by the udachnaya kimberlite pipe yakutia siberia radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2 9 ga pearson et al 1995 snyder et al in preparation o isotopic compositions generally show equilibration between the eclogitic minerals cpx grt 0 11 0 41 and have 18 o smow for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5 7 0 7 however several eclogites indicate significant deviations from this range at higher values of 6 8 7 0 also two eclogites lie at the lower end of the mantle range at values of 4 8 and 5 0 all in clinopyroxene c isotopic compositions of diamonds all have 13 c pdb in the range of 1 to 7 and are centered at approximately 5 also within the range of accepted mantle values the oh contents of the garnet are generally between 0 and 22 ppm as h 2 0 although two samples exceed 70 ppm this range of oh is similar to eclogitic garnet from the kaapvaal craton of southern africa the mantle c isotopic values of associated diamonds the majority of o isotopic data and the low oh content of the minerals although suggesting a general lack of crustal participation in the formation of the udachnaya eclogites do not rule out the participation of some ancient crustal material that these eclogites include both 18 o enriched and 18 o depleted types suggests that the protoliths may have been overprinted by both low and high temperature hydrothermal events cf jacob et ai 1994 a positive correlation between 18 o and 87 sr 86 sr allows the interpretation of these eclogites as representing a cross section of an archean ophiolite however the lack of a single coherent grouping on a plot of 18 o vs 87 sr 86 sr suggests that if the udachnaya eclogites were derived from oceanic crust they cannot be cogenetic and must represent at least two separate ophiolite sequences conversely if the eclogites are found to be cogenetic then a totally different process affected their formation and a probable metasomatic mechanism was operative because of the total lack of correlation of 18 o with other geochemical parameters we find no compelling evidence that all eclogites are derived ultimately from oceanic crust

exsolution of garnet within clinopyroxene of mantle eclogites major and trace element chemistry eclogite xenoliths from the mantle have experienced a wide variety of processes and p t conditions many of which are recorded in the mineral compositions and textures exsolution of garnet from clinopyroxene is one such texture occurring in a minority of mantle eclogites new analyses of clinopyroxene and garnet of eclogite xenoliths from kimberlites at bellsbank south africa and obnazhennaya yakutia russia are presented here and these are combined with data from the literature exsolution of garnet from clinopyroxene is generally lamellar although lens shaped garnets are also present major and trace element characteristics show a wide range of compositions and include eclogite groups a b and c rare earth element ree concentrations of garnet and pyroxene were determined by sims and the ree patterns are subtly different from those in ordinary eclogites differences include the absence of prominent eu anomalies in samples of this study and differences in the slopes of chondrite normalized ree patterns it is possible that these signatures are unique to exsolved eclogites a result of subsolidus elemental partitioning during exsolution some reconstructed whole rock compositions are aluminuous comparison with ordinary eclogites shows only minor differences implying a similar origin if the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite the excess aluminium was tied up in tschermak s molecule although the occasional presence of kyanite exsolution lamellae is indicative of a ca eskola component reconstructed pyroxenes from kyanite and corundum rich samples contain unrealistic amounts of aluminium for mantle pyroxenes a protolith or parental pyroxene threshold of 24 al2o3 may exist above which as in a plagioclase cumulate the final assemblage is kyanite and or corundum bearing

comment on mantle eclogites evidence of igneous fractionation in the mantle by j r smyth f a caporuscio and t c mccormick

3 d x ray tomography of diamondiferous mantle eclogite xenoliths siberia a review abstract diamonds form over billions of years hundreds of kilometers beneath the earth s surface and in combination with inclusions trapped within provide important constraints on the evolution of the mantle over geological time diamonds are generally studied as individual crystals sourced from highly explosive kimberlite pipes which entrain and subsequently disaggregate mantle fragments xenoliths en route to the surface this has resulted in a general absence of robust textural descriptions of diamonds relative to their hosting mantle protolith the textural associations of diamonds within their mantle host rocks are reviewed here on the basis of a compilation of x ray tomographic data for 17 diamondiferous eclogite xenoliths from siberian kimberlites this review represents a comprehensive description of diamonds relative to their host silicates the lack of such descriptions in previous studies is largely due to the rarity of these xenoliths the difficulty in preparing petrographic thin sections containing diamonds and their high monetary value high resolution computed x ray tomography hrcxt produces up to 1200 sequential 2 d slices through individual xenoliths each of which represents a pseudo thin section with a resolution on the order of 5 20 m the improved resolution of x ray imaging in recent studies allows for the identification of not only primary minerals but metasomatic minerals assemblages including spongy textured clinopyroxene phlogopite k richterite and hercynitic spinel allowing for the delineation of distinct metasomatic pathways through the xenoliths and their relationship to diamonds diamonds are observed in three distinct textural settings potentially representing several temporally distinct diamond growth events these setting includes 1 diamonds completely enclosed in garnet 2 diamonds associated with highly embayed silicate grain boundaries and 3 diamonds contained within distinct metasomatic plumbing systems diamonds observed completely enclosed in garnets suggest an early diamond forming event prior to major re crystallization and eclogite formation during subduction the occurrence of diamond in association with embayed garnets suggests that diamond grew at the expense of the hosting silicate protolith in addition the spatial relationships of diamonds with metasomatic pathways which are generally interpreted to result from late stage proto kimberlitic fluid percolation indicate a period of diamond growth occurring close to but prior to the time of kimberlite emplacement furthermore the paragenesis of sulfides within eclogite xenoliths are described using 3 d models for entire xenoliths volumes providing important constraints of the timing of sulfide mobilization within the mantle three d animations created using x ray tomography data for ten of the xenoliths can be viewed at the following link http eps utk edu faculty taylor tomography php

upper oligocene lower miocene gangrinboche conglomerate in the xigaze area southern tibet implications for himalayan uplift and paleo yarlung zangbo initiation abstract the gangrinboche conglomerate exposed along the yarlung zangbo suture zone records a crucial stage of the himalayan orogeny the type section of these strata in the xigaze area southern tibet including the qiuwu and dazhuka formations was studied by integrated stratigraphic sedimentologic petrographic and geochemical techniques palynological data and detrital zircon u pb ages indicate that the qiuwu formation was deposited during the latest oligocene to the earliest miocene most probably 23 ma while the overlying dazhuka formation was deposited during 23 18 ma the qiuwu formation was deposited in a deltaic setting and detritus was entirely derived from the gangdese magmatic arc in the north the dazhuka formation in contrast was deposited in mainly braided river environments and contains clasts derived from both the gangdese arc in the north and the himalayan orogen in the south clasts derived from the south first occur at the base of the dazhuka formation and increase in abunda

graphite and diamond bearing eclogite xenoliths from the bellsbank kimberlites northern cape south africa one diamond bearing and eight graphite bearing eclogite xenoliths are described from the bellsbank kimberlites cape province south africa graphite mostly occurs as discrete grains which are commonly in the form of tabular prisms diamond is octahedral both group i and group ii eclogite varieties are represented by the graphite bearing specimens while the single diamond bearing eclogite is of the group i variety the carbon isotopic composition of the graphite varies from 13c 7 to 13c 2 8 this is within the range of carbon isotopic compositions for inclusion free diamonds in kimberlite from this locality suggesting that the carbon for the eclogites as well as some of the kimberlite diamonds are derived from the same source the present day nd isotopic compositions of clinopyroxene from three graphite bearing xenoliths are slightly higher than the bulk earth estimate sr isotopic compositions of the clinopyroxene in these xenoliths vary from 87sr 86sr 0 703 to 87sr 86sr 0 706 this could be due to derivation of the xenoliths from a protolith with variable 87sr 86sr isotopic composition or could be the result of mixing between a low sr high 87sr 86sr component and a high sr low 87sr 86sr component

coupled climate and sea level changes deduced from huon peninsula coral terraces of the last ice age abstract huon peninsula papua new guinea is a tectonically unstable uplifting shoreline ringed by emergent coral terraces the terraces were formed during episodes of rapid sea level rise when corals constructed large discrete coral platforms that were subsequently uplifted uranium series ages of four prominent huon peninsula last glacial ois 3 coral terraces coincide with the timing of major north atlantic climate reversals at intervals of 6000 7000 yr between 30 000 yr and 60 000 yr ago terrace elevations when combined with uplift indicate 10 15 m high sea level excursions at these times we attribute the growth of the terraces directly to sea level rises arising from ice calving episodes from major north atlantic ice sheets and the antarctic ice sheet that precipitated extremes of cold climate called heinrich events these periods are associated with major discharges of land based ice and enhanced concentrations of ice rafted debris in deep sea cores sea levels at this time were 60 90 m lower than present

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

radiocarbon reservoir age of high latitude north atlantic surface water during the last deglacial the radiocarbon reservoir age of high latitude north atlantic ocean surface water is essential for linking the continental and marine climate records and is expected to vary according to changes in north atlantic deep water nadw production measurements from this region also provide important input and or tests of oceanic radiocarbon using 3 d global ocean circulation models here we present a surface water radiocarbon reservoir age record of the high latitude western north atlantic for the deglacial period via the use of fossil cold water corals growing in waters that are rapidly exchanged with nearby surface waters the reservoir age of high latitude north atlantic surface waters was computed from the radiocarbon age difference between our radiocarbon calibration record http radiocarbon ldeo columbia edu and our marine radiocarbon data 230th 234u 238u dates provide the absolute coral ages our high latitude north atlantic ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and vedde ash radiocarbon dates from central and eastern north atlantic show modern values 380 140 year n 14 during the bolling and allerod warm period and a 200 year increase in reservoir age 590 130 year n 10 during the entire younger dryas yd cold episode the reservoir age then decreased to 270 20 year n 2 at the preboreal yd transition although the dates are too sparse for us to be confident in this estimate we are not able to resolve the timing of the transition to increased reservoir ages from the mid allerod to the yd due to the relatively small change and correspondingly large uncertainty in the estimates the atmospheric 14c record derived from our atmospheric radiocarbon record displays a 40 per mil increase from 12 900 to 12 650 cal years bp coincident with the shift to high reservoir ages in the early yd cold event intrusion of 14c depleted antarctic intermediate water aaiw to the high latitude north atlantic and reduction of nadw formation are possible causes for the coincident shift to high reservoir ages in the north atlantic surface ocean and increased atmospheric 14c during the beginning of the yd event

the evolution of yarlung tsangpo river constraints from the age and provenance of the gangdese conglomerates southern tibet abstract cenozoic conglomerates are exposed discontinuously along the length of the yarlung tsangpo suture zone on the southern margin of the gangdese arc these conglomerates the gangdese conglomerates herein record a crucial stage in the uplift and erosion histories of the southern tibet after the initial india asia collision in the mt kailas area the gangdese conglomerates strata consist of multiple sedimentary cycles and each cycle is a fining upward sequence that was deposited by alluvial fan braided river and delta systems whereas in the xigaze area the gangdese conglomerates strata comprise a coarsening upward sequence that was deposited by delta braided river and alluvial fan systems based on the detrital and igneous zircon u pb ages the depositional ages of the gangdese conglomerates are late oligocene to early pliocene ca 26 5 ma in the mt kailas area late oligocene to middle miocene ca 26 15 ma in the xigaze area and late oligocene to early miocene ca 26 19 ma in the zedong area paleocurrent measurements and provenance data i e conglomerate clast composition sandstone petrology and detrital zircon age indicate that the initial detritus of the gangdese conglomerates were entirely derived from the north mainly from the gangdese arc sediment resulting from denudation to the south the xigaze forearc basin the yarlung tsangpo suture zone and the northern margin of the indian plate first appeared by the early miocene ca 19 ma and subsequently increased in abundance gradually our new results together with previous data from the xigaze area reveal 3 major stages in the evolution of the yarlung tsangpo river system 1 the southward flowing stage ca 26 19 ma featured southward draining transverse rivers that transported materials from the gangdese arc southward southward paleocurrents in the gangdese conglomerates indicate a northern source 2 the westward flowing stage ca 19 15 ma developed due to the uplift of the suture zone and tethys himalaya to the south northward draining rivers began to develop and lakes resembling a string of beads formed and finally connected together initiating the westward flowing paleo yarlung tsangpo river westward paleoflows were recorded in the gangdese conglomerates 3 the eastward flowing stage ca 15 ma present resulted from differential uplift and denudation of the southern tibet which reversed the direction of the young yarlung tsangpo river by ca 15 ma the deposition of the gangdese conglomerates was controlled by eastward paleoflows at this point the modern eastward flowing yarlung tsangpo river system was established

extreme enrichment of high field strength elements in jericho eclogite xenoliths a cryptic record of paleoproterozoic subduction partial melting and metasomatism beneath the slave craton canada an unusual suite of zircon and rutile bearing eclogite xenoliths from the 172 ma jericho kimberlite northwest territories canada displays a peculiar geochemistry highlighted by extreme enrichment of high field strength elements most profound are the large positive nb and zr anomalies that are linked to a high modal abundance of niobian rutile and zircon this peculiar geochemistry combined with results from u pb geochronology requires a complex origin involving east dipping paleoproterozoic subduction and partial melting of oceanic crust beneath the slave craton one or more metasomatic overprints prior to kimberlite entrainment and a mesoproterozoic thermal disturbance in the slave lithospheric mantle possibly linked to mackenzie igneous events

annual cycles of u ca in coral skeletons and u ca thermometry abstract we have discovered annual variations in the 238 u 40 ca ratio of porites coral skeletons measurements were made using thermal ionization mass spectrometric techniques yielding precisions of 2 2 for 238 u 40 ca and l 2 for 88 sr 40 ca coralline aragonite subsamples weighed 2 mg enabling submillimeter sampling resolution corresponding to monthly temporal resolution the annual nature of the cycles was confirmed by comparison with annual banding observable in x radiographs for two modern and one fossil sample the amplitude of the u ca variation ranges from 6 to 23 well outside of analytical error as annual u ca cycles appear to be a general feature of primary coralline aragonite the preservation of such features will be important in identifying unaltered coral for u series dating studies u ca variations mimic and are in phase with annual variations in 88 sr 40 ca for a given fractional shift in sr ca the fractional shift in u ca is about 6 times larger for the two modern corals 238 u 40 ca is strongly anticorrelated with measured temperature suggesting that 238 u 40 ca has potential as a paleothermometer if temperature is the only significant control on coralline 238 u 40 ca we reach the following conclusions from analyses of fossil samples a vanuatu sample which grew halfway through the last deglaciation gives u ca temperatures 4 to 5 c below modern values u ca thermometry applied to published data for barbados corals indicates that 1 temperature generally correlates with sea level 2 glacial temperatures stages 2 and 4 were 4 to 6 c lower than interglacial temperatures stages 7a 5e and 1 and 3 temperatures rose from glacial to interglacial values early in the last deglaciation thermometry applied to papua new guinea corals indicates that 1 temperatures were 5 to 6 c lower than interglacial temperatures from 13 to 10 ky bp then rose to present values 9 ky bp 2 the temperature depression between 13 and 10 ky bp is consistent with low temperatures observed in vanuatu during the same time interval and 3 the papua new guinea deglacial temperature history differs from that of barbados the results generally support estimates of tropical temperatures obtained from sr ca thermometry and snow line elevation data but disagree with those based on foram transfer functions a thermodynamic model suggests that coralline 238 u 40 ca may also be sensitive to marine carbonate ion concentration raising the possibility that some of the observed glacial interglacial 238 u 40 ca variation may result from glacial interglacial carbonate ion changes however the key experiments that might establish a coralline 238 u 40 ca carbonate ion relationship have yet to be performed

tracing patterns of erosion and drainage in the paleogene himalaya through ion probe pb isotope analysis of detrital k feldspars in the indus molasse india abstract the indus molasse is a pre and syn tectonic sedimentary sequence situated in the indus suture zone of the western himalaya spanning in time the collision of india and asia this deposit is well placed to record the evolving uplift and erosion history of the early himalayan orogen nd isotope analyses from clay extracted from shales interbedded within the dominantly alluvial sequence indicate a low negative nd 1 64 to 0 72 in the basal paleocene chogdo formation slightly more negative than measured values from the transhimalaya and kohistan dras arc up section nd becomes more negative as low as 10 05 indicating influence of a different more enriched source ion microprobe pb isotopic analyses of single k feldspars help constrain this source as being either the lhasa or karakoram blocks with westward paleo current flow favoring the former 207pb 204pb ratios are too low to be consistent with known indian plate sources a conclusion supported by the lack of muscovite or garnet that would be indicative of a high himalayan contribution given the known age of rapid cooling of the high himalaya at 20 ma and the lack of exposure of suitable lithologies prior to that time an age of sedimentation prior to 20 ma is inferred the post collisional change in paleo flow and provenance is suggested to reflect the initiation of the indus river during the early eocene this study demonstrates the power of combined bulk sediment and single grain analyses in resolving provenance in tectonically complex settings

stratigraphy of the upper cretaceous and lower tertiary strata in the tethyan himalayas of tibet tingri area china the 1500 m thick marine strata of the tethys himalaya of the zhepure mountain tingri tibet comprise the upper albian to eocene and represent the sedimentary development of the passive northern continental margin of the indian plate investigations of foraminifera have led to a detailed biozonation which is compared with the west tethyan record five stratigraphic units can be distinguished the gamba group upper albian lower santonian represents the development from a basin and slope to an outer shelf environment in the following zhepure shanbei formation lower santonian middle maastrichtian outer shelf deposits continue pebbles in the top layers point to beginning redeposition on a continental slope intensified redeposition continues within the zhepure shanpo formation middle maastrichtian lower paleocene the series is capped by sandstones of the jidula formation danian deposited from a seaward prograding delta plain the overall succession of these units represents a sea level high at the cenomanian turonian boundary followed from the turonian to danian by an overall shallowing upward megasequence this is followed by a final transgression regression cycle during the paleocene and eocene documented in the zhepure shan formation upper danian lutetian and by upper eocene continental deposits the section represents the narrowing and closure of the tethys as a result of the convergence between northward drifting india and eurasia the plate collision started in the lower maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift stronger subsidence and deposition took place from the middle maastrichtian to the lower paleocene the final closure of remnant tethys in the tingri area took place in the lutetian

oxygen isotopes in subducted oceanic crust a new perspective from siberian diamondiferous eclogites 1 there are three methods of investigating the oxygen isotope composition of oceanic crust 1 data from drilling obtained samples 2 study of obducted ophiolites and 3 from crustal derived xenoliths however each approach has limitations drilling capabilities are limited to the upper 1 3 km of the oceanic crust ophiolites are segments of oceanic crust and underlying mantle exposed at the earth s surface but their origin and emplacement are still debated nonetheless the study of ophiolite sequences has enabled many of the fundamental observations regarding ancient oceanic crust and the oxygen isotope signature of the upper mantle analyses of ophiolite sequences e g samail reveal 18o variations ranging from isotopically enriched in relation to morb 18o morb to isotopically depleted low 18o morb interpreted to show the effects of seawater interaction at different temperatures mantle derived xenoliths also display compelling geochemical evidence suggesting they are remnants of subducted ocean crust and may in fact be more representative samples some kimberlite hosted diamondiferous eclogite xenoliths with crustal protoliths reveal a marked oxygen isotope disparity between oceanic crust and the upper mantle i e a bimodal distribution of both mantle like 18o 5 6 and isotopically enriched oxygen 18o 6 8 the isotopically high 18o values are often observed in eclogite xenoliths with ultramafic protoliths which form beyond the low temperature alteration zone this finding is in contrast to traditional oxygen isotope versus depth curves of ophiolite sequences here we present a compiled data set from siberian eclogites and pyroxenites that illustrates this bimodality using magnesium number mg as a depth proxy and investigate the conditions that could potentially result in the observed abnormal oxygen isotope variations

oxygen isotope composition of syngenetic inclusions in diamond from the finsch mine rsa oxygen isotope data have been obtained for silicate inclusions in diamonds and similar associated minerals in peridotitic and eclogitic xenoliths from the finsch kimberlite by laser fluorination oxygen isotope analyses of syngenetic inclusions weighing 20 400 g have been obtained by laser heating in the presence of clf3 18o 16o ratios are determined on oxygen converted to co2 over hot graphite and for samples weighing less than 750 g producing 12 moles o2 enhanced co production in the graphite reactor causes a systematic shift in both 13c and 18o that varies as a function of sample weight a pressure effect correction procedure based on the magnitude of 13c co2 depletion relative to 13c graphite is used to obtain corrected 18o values for inclusions with an accuracy estimated to be 0 3 for samples weighing 40 g syngenetic inclusions in host diamonds with similar 13c values 8 4 to 2 7 have oxygen isotope compositions that vary significantly with a clear distinction between inclusions of peridotitic 4 6 to 5 6 and eclogitic paragenesis 5 7 to 8 0 the mean 18o composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle 5 25 0 22 whereas syngenetic purple garnet inclusions possess relatively low 18o values 5 00 0 33 reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of cr rich garnet and or exchange with the diamond forming fluid garnet in eclogite xenoliths in kimberlite show a range of 18o values from 2 3 to 7 3 but garnets in diamondiferous eclogites and as inclusions in diamond all have values 4 7

nd isotopic evolution of the precambrian mantle abstract the most convincing examples of proterozoic oceanic crust assemblages occur in the circum superior fold belt of quebec nd isotope analyses of the chukotat and hellancourt basalts from this belt constrain the e nd of the early proterozoic depleted mantle reservoir at 4 to 6 along with data for phanerozoic ophiolites and modern morb the proterozoic basalts infer a constant rate of evolution of the depleted mantle at a rate of 2 2 e nd units per ga recalculation of sm nd isotopic data for archaean greenstone belts to their most reasonable pb pb or u pb ages suggests significantly greater isotopic heterogeneity in the archaean presumed uncontaminated tholeiites and al undepleted komatiites fall within the extrapolated growth limits for the proterozoic phanerozoic depleted mantle while al depleted komatiites adk in early archaean greenstone belts have significantly higher e nd the latter require derivation from a source whose isotopic composition evolved at 2 4 times the rate of the al undepleted komatiite and tholeiite source the isotopic heterogeneities noted for the archaean indicate a compositionally layered mantle by the late archaean the adk source was either isolated by declining geothermal gradients or had become remixed with the overlying mantle giving rise to the present depleted mantle reservoir

eclogitic inclusions in diamonds evidence of complex mantle processes over time the first ion probe trace element analyses of clinopyroxene garnet pairs both included within diamonds and from the eclogite host xenoliths are reported these diamondiferous eclogites are from the udachnaya and mir kimberlite pipes yakutia russia the major and trace element analyses of these diamond inclusion and host rock pairs are compared in order to determine the relative ages of the diamonds confirm or deny genetic relationships between the diamonds and the eclogites evaluate models of eclogite petrogenesis and model igneous processes in the mantle before during and after diamond formation the most striking aspect of the chemical compositions of the diamond inclusions is the diversity of relationships with their eclogite hosts no single distinct pattern of variation from diamond inclusion minerals to host minerals is found for all four samples garnet and clinopyroxene inclusions in the diamonds from two samples u 65 3 and u 66 3 have lower mg s lower mg and higher fe contents and lower lree than those in the host eclogite we interpret such variations as due to metasomatism of the host eclogite after diamond formation one sample u 41 3 shows enrichment in diamond inclusion mree enrichment relative to the eclogite host and may indicate a metasomatic event prior to or during diamond formation bulanova 2 found striking differences between inclusions taken from within different portions of the very same diamond clinopyroxene inclusions taken from the central early portions of yakutian diamonds were lower in mg and mg contents by up to 25 than those later inclusions at the rims of diamonds these trends are parallel to those between diamond inclusions and host eclogites determined for four of the five samples from the present study and may merely represent changing magmatic and or p t conditions in the mantle garnet trace element compositions are similar in relative proportions but variable in abundances between diamond inclusions and host eclogites this is probably due to the rapid diffusion of trace elements in garnet under mantle temperatures and consequent alteration of the garnet and not due to juvenile diamonds locking in source heterogeneities c f 3 trace element compositions of clinopyroxenes included in diamonds are generally similar to those in the host eclogite however one host clinopyroxene does show enrichment in the lree compared to that in the inclusion and may be attributed to mantle metasomatism not related to kimberlite transport in another eclogite m 46 the host clinopyroxene is depleted in the lree and fe and enriched in the hree and mg relative to the inclusion and is consistent with partial melting of the eclogite subsequent to diamond formation smnd ratios in clinopyroxenes appear to be little affected by these processes for most samples allowing sm nd isotopic studies to yield important information about ancient protoliths eclogitic mineral inclusions in yakutian diamonds appear consanguineous with the diamonds a contention supported by the observations of bulanova 2 therefore re os whole rock and smnd clinopyroxene age determinations of the udachnaya eclogites also yield the time of diamond formation approximately 2 9 ga 32 33

early proterozoic oceanic crust and the evolution of subcontinental mantle eclogites and related rocks from southern africa seven eclogitic nodules from kimberlites in southern africa have been studied in detail for whole rock major and trace element geochemistry petrography and mineral chemistry by electron microprobe high purity mineral separates from six of these samples have been analyzed for trace elements and for the isotopic composition of sr nd and oxygen three eclogite groups are recognized group a eclogites have very high mg fe low na in pyroxene moderate 18o 4 7 to 5 3 and low 87sr 86sr and 143 144nd olivine and enstatite may also be present as accessory phases in this group group b eclogites have moderate to high mg fe ratios high na in pyroxene low 18o 3 0 to 3 4 high 87sr 86sr and 143nd 144nd ratios and extremely lree depleted hree enriched garnets with no eu anomalies group c eclogites have low mg fe ratios high na in pyroxene and variable sr nd and o isotope compositions accessory feldspar is present in one sample that may be of crustal origin mineral separates of garnet and pyroxene have positive eu anomalies the group a eclogites are too refractory to represent magma compositions and must have formed as cumulate dike rocks in the upper mantle that contain a minor trapped liquid component this is supported by the presence of accessory olivine and enstatite and by their ksm and knd which are similar to empirical pyroxene garnet partition coefficients the group b eclogites are extremely depleted in incompatible elements and have ndvalues 10x to 20x morb the high 87sr 86sr of these eclogites is not consistent with their strongly depleted lree and high nd and cannot be primary it must have been imposed on the protolith the low 18o of these eclogites cannot form by mantle fractionation processes and must also be inherited from the protolith both the sr and oxygen isotope data are consistent with high temperature hydrothermal alteration of a basaltic protolith followed by partial melting to form the refractory compositions observed now the hydrothermal fluid may have been sea water but secondary enrichment of the protolith in rb is required to generate the observed high sr ratios the high na content of the pyroxenes supports a spilitic alteration event the major and trace element characteristics of the group c eclogites are consistent with recrystallization of a cumulate gabbro protolith one group c rock is probably a garnet granulite derived from the lower crust the other may represent the plutonic portion of oceanic crust a reconstructed whole rock isochron for the three bellsbank eclogites yields an age of 2 1 0 1 b y this implies that plate tectonic processes involving the generation and subduction of oceanic crust have been active since the early proterozoic the early proterozoic ocean crust consisted of two components a plutonic section of gabbro cumulates group c eclogites and a volcanic section of basalt group b eclogites the volcanic component has undergone substantial hydro thermal alteration and subsequent partial melting to form a refractory residue the plutonic component is less modified

the origins of yakutian eclogite xenoliths owing to the association with diamonds eclogite xenoliths have samples were derived from typical depleted mantle however two groups of three samples each indicate both enriched mantle and received disproportionate attention given their low abundance in kimberlites several hypotheses have been advanced for the origin of possible ultra depleted mantle present beneath yakutia during the late archean and early proterozoic the vast majority of eclogites eclogite xenoliths from the subduction and high pressure melting of oceanic crust to cumulates and liquids derived from the upper studied from the obnazhennaya pipe also exhibit characteristics of mantle we have amassed a comprehensive data set including group a eclogites and are probably derived directly from the mantle majorand trace element mineral chemistry carbon isotopes in however the eclogites from the mir kimberlite are more typical of diamonds and rb sr sm nd re os and oxygen isotopes in other eclogites world wide and show convincing evidence of a recycled ultrapure mineral and whole rock splits from eclogites of the udoceanic crustal affinity we concur with the late ted ringwood achnaya kimberlite pipe yakutia russia furthermore eclogites that eclogites can be formed in a variety of ways both within the from two other yakutian kimberlite pipes mir and obnazhennaya mantle and from oceanic crustal residues have been studied in detail and offer contrasting images of eclogite protoliths relative to eclogites from southern africa and other yakutian localities udachnaya eclogites are notable in the absence of chemical zoning in mineral grains as well as the degree of light

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

comment on extending the radiocarbon calibration beyond 26 000 years before present using fossil corals by t c chiu r g fairbanks r a mortlock a l bloom quaternary science reviews 24 2005 1797 1808

india asia convergence insights from burial and exhumation of the xigaze fore arc basin south tibet the composite fore arc syncollisional xigaze basin in south tibet preserves a key record of india asia collision new apatite fission track and zircon u th he data from an n s transect across the preserved fore arc basin sequence near xigaze show a consistent northward late cretaceous to middle miocene younging trend while coexisting apatite u th sm he ages are all miocene corresponding detrital zircon u pb data are also reported for constraining the cretaceous depositional ages of the xigaze basin sequence in the region thermal history modeling indicates that the basin experienced northward propagating episodic exhumation along with a northward migration of the depocenter and a pre existing cenozoic syncollisional basin sequence which had been removed in the southern part fore arc exhumation commenced in the late cretaceous 89 2 ma following transition to a syncollisional basin in the paleocene sedimentation in the central and northern xigaze basin continued until the latest eocene 34 4 ma ongoing folding and thrusting e g great counter thrusts caused by progressive plate convergence during late oligocene early miocene time resulted in regional uplift and considerable basin denudation which fed two fluvial basins along its northern and southern flanks and exposed the basement ophiolite subsequent incision of the yarlung river resulted in miocene cooling in the region different episodes in the exhumation history of the xigaze basin caused by thrusting of an accretionary wedge and ophiolitic basement can be linked to changes in india asia convergence rates and the changing subduction pattern of the indian and neo tethyan slabs

new constraints on the india asia collision the lower miocene gangrinboche conglomerates yarlung tsangpo suture zone se tibet abstract lower miocene conglomerates crop out along the length of the yarlung tsangpo suture zone on the southern margin of the lhasa terrane these conglomerates known by various local names are correlated herein as the gangrinboche conglomerates all units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the lhasa terrane is ubiquitous at most localities the tops of sections are either removed by erosion or truncated by north directed thrusts these conglomeratic molasse units developed in response to the india asia collision and record aspects of its development in all units initial clast derivation was from the lhasa terrane on the northern margin of the yarlung tsangpo suture zone up section the first appearance of clasts derived from terranes within the suture zone and the northern margin of india all of which lie to the south of any outcrops of gangrinboche conglomerates is observed although these units were previously thought to be eocene analysis of fossil and structural constraints indicates early miocene deposition as development of the gangrinboche conglomerates records a significant phase in the evolution of the india asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models

comment on trace element crystal chemistry of mantle eclogites by f a caporuscio and j r smyth caporuscio and smyth have presented crystal chemical study on eleven mantle derived eclogites from the bellsbank 8 and roberts victor 3 kimberlites south africa they combine these results with experimental partition coefficients and geochemical modelling to argue for a high pressure igneous cumulate origin from a morb like melt for these eclogites in particular they highlight the kyanite bearing eclogites grospydites especially the presence of a middle rare earth element mree enrichment which may also be considered in terms of lree and hree depletions caporuscio and smyth as well as smyth et al cite this phenomenon as evidence for an origin by igneous accumulation of hyperaluminous clinopyroxene at high pressure however this type of ree pattern has also been interpreted as depicting a positive eu anomaly inherited from a subducted plagioclase rich oceanic crustal protolith shervais et al taylor and neal neal et al

discovery and significance of cretaceous fossils from the xigaze forearc basin tibet abstract the xigaze cretaceous forearc basin located between the lhasa block and indus yarlung suture was filled with flysch deposits during the mid cretaceous as the tethys oceanic crust was being subducted underneath eurasia the flysch comprises the xigaze group dominated by the angren formation well preserved newly discovered foraminifera within the angren formation constrain its age to late albian to late coniacian radiometric dating of the ophiolite belt impinging on the southern edge of the forearc basin yielded an age of 120 10 ma which implies that the deposition in the forearc basin occurred shortly after initiation of the subduction of the neotethys

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

analysis of the atmospheric 14c record spanning the past 50 000 years derived from high precision 230th 234u 238u 231pa 235u and 14c dates on fossil corals abstract changes in the geomagnetic field intensity solar variability and the internal changes of the carbon cycle are believed to be the three controlling factors of past atmospheric radiocarbon 14 c concentrations denoted as 14 c of these three it is believed that the field intensity is the dominant factor we analyze an atmospheric 14 c record spanning the past 50 000 years based on previously published 230 th 234 u 238 u and 14 c dates of fossil corals from kiritimati barbados araki and santo islands and identify the role of the laschamp geomagnetic field excursion on the long term trend of the 14 c record there is a general consistency between the coral 14 c record and the 14 c output from carbon cycle models based on the global 14 c production estimates high precision high accuracy 230 th 234 u 238 u dates and redundant 231 pa 235 u dates anchor the timing of this 14 c record we propose that a significant fraction of the long term 14 c trend may be due to inaccuracies in the generally accepted 14 c decay constant the uncertainty in estimating the shape of 14 c beta spectrum below 20 kev leads to one of the greatest errors in decay constant estimates once the 14 c half life is validated via redundant techniques 14 c records will provide a better opportunity to examine the roles of carbon cycle and 14 c production influences

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

ultradeep greater than 300 kilometers ultramafic upper mantle xenoliths geophysical discontinuities in earth s upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid state dissolution of pyroxene into garnet with increasing depth these predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet bearing xenoliths from a diamondiferous kimberlite conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars placing the origin of these xenoliths at depths of 300 to 400 kilometers these ultradeep xenoliths support the theory that the 400 km seismic discontinuity is marked by a transition from peridotite to eclogite

a new siluro devonian paleopole from early paleozoic rocks of the franklinian basin north greenland fold belt demagnetization of dolomitic shales dolomites and carbonate breccias of the cambro ordovician kap stanton formation within the franklinian basin sequence of the north greenland fold belt yields a characteristic direction declination inclination is 16 16 tilt corrected from 14 of the 28 sites a negative breccia test and positive local and regional fold tests bracket the age of the magnetization as postdepositional but pre ellesmerian pre carboniferous after rotating the corresponding paleopole 1 n 302 e to account for the cretaceous tertiary opening of the labrador sea the greenland pole remains removed by approximately 20 from the existing north american apparent polar wander path apwp however the rotated paleopole 2 s 284 e in north american coordinates is similar to british silurian and devonian paleopoles in a bullard et al 1965 reconstruction pole is 2 s 322 e in british coordinates we interpret our new paleopole as late silurian to early devonian in age requiring a modification of previously published apwps for laurentia if the age assignment is correct the new paleopath for north america greenland and that for great britain can be brought into good agreement by closing the atlantic ocean thus indicating that north america greenland and great britain were assembled in a supercontinent in late silurian to early devonian time

fluid inclusion and mineral isotopic compositions hco in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism eclogite facies metagabbros from the monviso ophiolitic complex italian western alps provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of 40 km we have determined the omphacites 18o and the abundance d and 13c of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90 fluid loss the remaining fluid was trapped as primary water rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone oxygen isotopes of omphacite omp and hydrogen isotopes of water in the fluid inclusions fi analyzed in low strain rocks mylonites and undeformed deformed veins show marked variations 18oomp and dfi values ranging from 3 0 to 5 3 and from 31 to 93 respectively detailed isotopic analysis of several individual vein wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter therefore the eclogitic minerals and fluids filling the veins are concluded to be locally derived our results argue against recent models which suggest large scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism on average the h2o contents and dfi value are within the upper mantle range the carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 13c value of 24 2 1 2 two carbonaceous components can be recognized condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions 18oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole rock values from hydrothermally altered oceanic crust it is suggested that the isotopic imprint of the monviso eclogitic minerals and fluids represents the signature of mid ocean ridge hydrothermal alteration and subduction zone eclogitization processes the rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths in addition the range in 18oomp values recorded in the different microstructural domains covers most of the 18o values reported in type a b and c eclogitic xenoliths implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths

rigorous quality control criteria for screening coral samples and radiocarbon calibration data based on 14c 230th 234u 238u and 231pa 235u dated corals a reply to the comment by yusuke yokoyama and tezer m esat on extending the radiocarbon calibration beyond 26 000 years before present using fossil corals by t c chiu r g fairbanks r a mortlock a l bloom quaternary science reviews 24 2005 1797 1808

diamondiferous eclogites from yakutia siberia evidence for a diversity of protoliths major element and ree compositions of 14 diamondiferous eclogites from the udachnaya kimberlite in yakutia siberia have been determined by electron microprobe and secondary ion mass spectrometer sims based on previous clinopyroxene classification schemes e g taylor and neal 1989 all of these eclogite xenoliths belong to group b c although some of the garnet compositions and mineral ree abundances are inconsistent with the indicated groups this demonstrates the inadequacy of the classification scheme based on african eclogites for application to siberian samples because of the coarse grain size of the udachnaya nodules meaningful modal abundances could not be obtained however reconstructed ree compositions using various garnet clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages many of these reconstructed ree compositions show lree depletions some depletions are consistent with an origin either directly or through partial melting as normal or type i ocean floor basalt others however require material of eclogitic or pyroxenitic affinities to undergo partial melting this facilitates the depletion of lree while leaving the hree at nearly original levels many of the eclogites of south africa are consistent with a protolith of anomalous or type ii ocean floor basalt this fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification

palynological indications of environmental changes during the late creataceous eocene on the southern continental margin of laurasia xizang tibet abstract a palynostratigraphic and palynofacies study of a geological section at cuojiangding in zhongba county has provided a basis for discussing the palaeoenvironmental evolution of the southern continental margin of laurasia in xizang tibet towards the end of the cretaceous period and during the paleogene it is the only record of fossil spores and pollen grains from collision and convergence related sediments close to the major yarlung zangbo suture deposits of the padana formation at the base of the section through the qubeiya quxia and jialazi formations to the rujiao zangbo conglomerate at the top are considered to range in age from santonian to eocene or possibly oligocene during this period the india plate collided with and continued to push against laurasia plate two spore and pollen zones and three palynofacies types are recognized these are correlated with two transgressive regressive cycles associated with the collision periods of geologically rapid uplift being reflected by the molasse like sediments of the upper quxia formation and the rujiao zangbo conglomerate which accumulated during the late paleocene and eocene oligocene respectively the first regression led to intermittent subaerial exposure and erosion of the cuojiangding area the second ended the marine history of the area and led first to the development of swamps in a subtropical climate now preserved in the coal bearing qiuwu formation and later to the development of mountainous terrain with a cooler climate at higher elevations

stable and radiogenic isotope study of eclogite xenoliths from the orapa kimberlite botswana eclogite xenoliths from orapa can be accurately classified as group i or group ii on the basis of na2o in garnet and k2o in clinopyroxene group i xenoliths are commonly diamondiferous while group ii xenoliths are diamond free both xenolith varieties may contain graphite isotopic character is to some degree correlated with major and trace element chemistry group ii samples with ca poor garnet have clinopyroxenes with radiogenic 87sr86sr 0 705 0 709 and the least radiogenic 143nd144nd 0 5122 0 5125 group i eclogites with higher ca and fe in garnets have less radiogenic 87sr86sr 0 702 0 7066 and bulk earth or higher 143nd144nd ratios group i eclogites have more radiogenic 206pb204pb 18 6 19 than group ii xenoliths 16 5 18 6 in contrast group ii xenoliths have more variable and in some cases more radiogenic 206pb204pb 36 6 39 3 than group i xenoliths 38 3 38 4 the sr sm nd and pb concentrations of minerals in orapa group i eclogite xenoliths are much lower than in group ii samples all the group ii xenoliths are inferred to be enriched in light rare earth elements while group i xenoliths are probably characterised in many cases by light rare earth element depletion constituent garnet and clinopyroxene in both group i and ii eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement mineral as well as calculated whole rock 143nd144nd compositions of most of the group i eclogites are too close to bulk earth and depleted mantle estimates in order to obtain useful model age information depleted mantle model ages derived from the much lower 143nd144nd compositions of the group ii eclogite xenoliths range from 661 to 1248 my with an average clinopyroxene model age of 908 my and an average whole rock model age of 1016 my on the basis of an observed covariation of o and sr isotopic compositions the entire orapa group i eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment the group ii xenoliths might have crystallised from a melt which derives from a protolith with time averaged lree depletion their radiogenic sr isotope character could be due to interaction of the melt with metasomatised lithosphere or might be a superimposed metasomatic signature

fingerprints and age models for widespread new zealand tephra marker beds erupted since 30 000 years ago a framework for nz intimate abstract the role of tephras in the nz intimate project is a critical one because most high resolution palaeoclimatic records are linked and dated by one or more tephra layers in this review first we document eruptive distributional and compositional fingerprinting data both mineralogical and geochemical for 22 key marker tephras erupted since 30 000 years ago to facilitate their identification and correlation we include new glass compositional data the selected marker tephras comprise 10 from taupo and nine from okataina volcanoes rhyolitic one from tuhua volcano peralkaline rhyolitic and one each from tongariro and egmont volcanoes andesitic second we use four approaches to develop 2 age models for the tephras youngest to oldest 1 calendar ages for kaharoa and taupo y were obtained by wiggle matching log derived tree ring sequences dated by 14c 2 whakaipo v was dated using an age depth model from peat 3 14 tephras in the montane kaipo peat sequence waimihia s unit k whakatane tuhua mamaku rotoma opepe e poronui c karapiti b okupata konini waiohau rotorua rerewhakaaitu were dated by simultaneously wiggle matching stratigraphic position and 51 independent 14c age points against intcal04 using bayesian probability methods via both oxcal and bpeat and 4 the five oldest tephras erupted before ca 18 000 cal yr bp were dated by calibrating limited numbers of 14c ages using intcal04 okareka or comparison curves of the expanded cariaco basin sequence te rere kawakawa oruanui poihipi okaia kawakawa oruanui tephra the most widely distributed marker tephra was erupted probably ca 27 097 957 cal yr bp potential dating approaches for the older tephras include their identification in antarctic ice cores if present or annually laminated sediments for which robust calendar age models have been constructed high precision ams 14c dating on appropriate material from environmentally stable sites systematic luminescence dating or new radiometric techniques e g u th he if suitable minerals are available and errors markedly reduced further application of bayesian age modelling to stratigraphic sequences of 14c ages possibly augmented with luminescence ages may help refine age models for pre holocene tephras with the largest errors finally we discuss the critical role these marker tephras play in the ongoing construction of an event stratigraphy for the new zealand region which is a key objective of australasian and southern hemisphere intimate projects