magnetic susceptibility as a high resolution correlation tool and as a climatic proxy in paleozoic rocks merits and pitfalls examples from the devonian in belgium low field magnetic susceptibility in measurements are quick and sensitive enabling the creation of high resolution records making in an attractive correlation tool and a proxy for paleoclimate and paleoenvironments in geologically young material foremost in cenozoic sediments in belongs to the geoscientist s toolkit however in is a convolved signal and may reflect other processes than the often implicitly inferred depositional conditions diagenesis remagnetization and low grade metamorphism can potentially obscure the original depositional in signal this aspect is particularly important when interpreting in records from paleozoic rocks here we review data obtained from a large sample collection of middle to upper devonian sections in belgium comparison of in trends with paleoenvironmental indicators facies and with detrital input proxies zr th ti al allowed to assess the persistence of depositional trends furthermore the in signal was deconvolved into its dominant mineralogical contributions with the help of magnetic property analysis the main results are pointing to a magnetic signal dominated by fine grained magnetite of which paleomagnetic analysis indicated a formation during carboniferous remagnetization this prompts a potentially strong influence of post depositional processes and it complicates the interpretation of in records in terms of depositional environments however in most of the sections there is a relatively good relationship between in trends and facies evolution and between in and geochemical proxies for detrital inputs this indicates that the newly formed magnetite grains would at least partly remain where they are formed and this allows a relative preservation of the original signal despite the strong influence of diagenesis two sections show a stronger impact of diagenesis where for about half of these sections the primary depositional information is lost the eifelian givetian monts de baileux section was selected for time series analysis of the in series the average spectral misfit asm method is applied to explicitly evaluate the null hypothesis of no orbital signal and in this section there is 99 05 chance that the ms signal is reflecting an orbital imprint

identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

initial pb sr nd isotopic heterogeneity in a single allanite epidote crystal implications of reaction history for the dating of minerals with low parent to daughter ratios we analyzed 17 fragments from a zoned allanite epidote crystal ca 2 2 mm 4 0 mm which had formed during different prograde and retrograde stages of ultra high pressure uhp and amphibolite facies metamorphism 240 230 ma sulu belt e china for the isotopic composition of pb nd and sr and contents of pb u and th sr and rb and nd and sm since most fragments had 238u 204pb and 232th 204pb values less than 1 corrections for in situ pb growth are small and uncertainties in the recalculation of the pb isotopic compositions to 240 ma are insignificant the recalculated pb falls on a linear trend in the 206pb 204pb vs 207pb 204pb diagram with the allanite defining the low 206pb 204pb end 17 07 of this trend and the epidote defining its high 206pb 204pb end 17 56 the recalculated data scatter in the 206pb 204pb vs 208pb 204pb diagram which implies that the initial pb isotopic variation reflects the involvement of at least three different pb sources the low 87rb 86sr values account for a change in 87sr 86sr by in situ 87sr growth of less than 0 0007 which implies that the isotopic heterogeneity of 87sr 86sr 0 70601 0 7200 is a primary feature the pb and sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product it is likely that such an initial isotopic heterogeneity also exists for nd but it could not be resolved in the present study initially heterogeneous pb and sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates especially for minerals with relatively low parent to daughter ratios

carbon isotopic changes in benthic foraminifera from the western south atlantic reconstruction of glacial abyssal circulation patterns oxygen and carbon isotopic analyses have been performed on the benthic foraminifer planulina wuellerstorfi in seven late quaternary cores from the vema channel rio grande rise region the cores are distributed over the water depth interval of 2340 to 3939 m which includes the present transition from north atlantic deep water nadw to antarctic bottom water aabw the carbon isotopic records in the cores vary as a function of water depth the shallowest and deepest cores show no significant glacial interglacial difference in 13c four of the five cores presently located in the nadw have benthic foraminiferal 13c that is lower during glacial isotopic stages based on bathymetric gradients in 13c we conclude that like today there were two water masses present in the vema channel during glacial intervals a water mass enriched in 13c overlying another water mass depleted in 13c the largest gradient of change of 13c with depth however occurred at 2 7 km 1 km shallower than the present position of this gradient on the basis of paleontologic and sedimentologic evidence we consider it unlikely that the nadw aabw transition shallowed to this level reduced carbon isotopic gradients between the deep basins of the north atlantic and pacific oceans during the last glaciation suggest that production of nadw was reduced lower production of nadw may have modified the local abyssal circulation pattern in the vema channel region

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

apparent correlation of palaeomagnetic intensity and climatic records in deep sea sediments most reports of a correlation between pleistocene climate and geomagnetic field intensity rely strongly on the assumption that sediment natural remanent magnetic nrm intensity provides a record of geomagnetic field strength and is not sensitive to local changes in properties of the sediment critical assessment of relevant data presented here and elsewhere1 2 from deep sea sediment cores shows that a pronounced dependence of nrm intensity on sediment composition can occur which implies that this assumption is unlikely to be generally valid as sediment composition often reflects varying depositional conditions induced by climatic change the significance of correlations proposed between pleistocene palaeomagnetism and climatic indicators in deep sea sediments may be less dramatic than sometimes supposed

allanite and other ree rich epidote group minerals epidote group minerals rich in rare earth elements ree in particular allanite are common accessory phases in igneous metamorphic metasomatic and sedimentary rocks small amounts of ree are present in most epidote group minerals but in allanite and the related minerals dissakisite ferriallanite dollaseite khristovite and androsite the ree are essential structural constituents an important characteristic of ree rich epidote group minerals is that their octahedrally coordinated m sites contain major amounts of divalent cations this paper summarizes literature data for these minerals and discusses their chemistry occurrence phase relations and petrologic and geologic significance the chapter emphasizes allanite because it is the most common and best studied of the ree rich epidote group minerals epidote group minerals contain isolated silicon tetrahedra and corner sharing groups of two tetrahedra and are thus assigned to the disilicate or sorosilicate structural family for a detailed description of the structure see franz and liebscher 2004 the epidote group structural formula is a2m3 sio4 si2o7 o f oh or in a simplified form a2m3si3o11 o f oh in which a ca sr pb2 mn2 th ree3 and u and m al fe3 fe2 mn3 mn2 mg cr3 and v3 deer et al 1986 there are two structurally different a sites a 1 and a 2 with different coordination numbers and there are three different m sites m 1 m 2 and m 3 which are all octahedrally coordinated ueda 1955 dollase 1971 in epidote group minerals trivalent ree are accommodated in the a sites which in endmember epidote both contain ca the incorporation of ree3 is commonly charge balanced by a divalent cation fe2 mn2 mg substituted for a trivalent one in the m sites table 1

magnetostratigraphy susceptibility of the přidolian lochkovian silurian devonian gssp klonk czech republic and a coeval sequence in anti atlas morocco abstract the magnetosusceptibility event and cyclostratigraphy msec record for the p idolian lochkovian silurian devonian global boundary stratotype section and point gssp klonk prague basin czech republic is described and used to establish a magnetostratigraphy susceptibility profile for the gssp gssp msec data are summarized into three magnetozones msz the tma msz late p idolian with 13 msszs mssz the klonk msz latest p idolian and earliest lochkovian with 17 msszs and the voskop msz early lochkovian with 7 msszs the base of klonk mssz 2 is coincident with the base of lochkovian and the devonian the proposed magnetostratotype for the p idolian lochkovian boundary is supported by ms data from a nearby core through the gssp sequence and into the upper ludlow three additional mszs are recognized for the portion of the core sequence older than the gssp boundary sequence the extension of the mszs and msszs away from the proposed magnetostratotype is tested by comparison with a lochkovian sequence in the western sahara of southeastern morocco mszs are the result of global sea level fluctuations that alter base level and the pattern of erosion and influx of detrital paramagnetic mineral components into the marine environment large scale transgressive and regressive patterns interpreted from the msec data establish that the silurian devonian boundary in the barrandian region falls between a short lived transgressive pulse in the latest p idolian and an equally short lived regressive pulse in the earliest lochkovian msszs identified in the gssp and core sequences are the result of variation in climate cycles and the resulting periodicity can explained by either the 38 9 ka silurian and devonian equivalent of the modern 54 ka obliquity cycle or the eccentricity cycle of 106 ka periodicity use of the obliquity periodicity gives a duration for the barrandian lochkovian of 4 46 ma which agrees with the most recent geochronometric estimate of 4 5 ma for the lochkovian use of the eccentricity periodicity gives a duration for the barrandian lochkovian of 12 16 ma which agrees with the previous geochronometric estimates of 12 2 and 12 5 ma for the lochkovian in any case the cyclic nature of the msec data established by the msszs can be explained by variations in the rate of supply of weathered terrigenous paramagnetic grains to the marine system these variations in supply occurred due to climate changes resulting from the either the obliquity or eccentricity of the earth s orbit knowledge of the periodicity of lochkovian cycles in the barrandian area allows estimation of the rates of sediment accumulation the duration of fossil ranges and the rates of evolution depending on the choice of obliquity or eccentricity to explain the cyclicity in the barrandian lochkovian the estimated rate of sediment accumulation in the gssp outcrop averaged either 51 or 140 ka m the range of the graptoloid monograptus uniformis angustidens wholly contained within the lochkovian sequence at the gssp is also estimated at either 51 or 140 ka

permian continental basins in the southern alps italy and peri mediterranean correlations the late carboniferous to permian continental successions of the southern alps can be subdivided into two main tectono sedimentary cycles separated by a marked unconformity sealing a middle permian time gap generally estimated at over 10 ma the lower cycle 1 between the variscan crystalline basement and the early permian is mainly characterised by fluvio lacustrine and volcanic deposits of calc alkaline acidic to intermediate composition which range up to a maximum thickness of more than 2 000 m the upper cycle 2 which is devoid of volcanics is mostly dominated through the mid late permian by alluvial sedimentation which covered the previous basins and the surrounding highs giving rise to the subaerial verrucano lombardo val gardena groden red beds up to about 800 m thick the palaeontological record from the terrigenous deposits of both the above cycles consists mainly of macro and microfloras and tetrapod footprints the age of the continental deposits is widely discussed because of the poor chronological significance of a large number of fossils which do not allow reliable datings however some sections are also controlled by radiometric calibrations the comparison with some selected continental successions in southern europe allows to determine their evolution and set up correlations a marked stratigraphic gap shows everywhere between the above mentioned cycles 1 and 2 as in the southern alps the gap reaches the greatest extent during the mid permian near the illawarra reversal geomagnetic event 265 ma in western europe however such as in provence and sardinia the discussed gap persists upwardly to late permian and early triassic or slightly younger times i e to the onset of the alpine sedimentary cycle even though in northeastern spain iberian ranges balearic islands this gap results clearly interrupted by late guadalupian lopingian deposits the above two major tectonosedimentary cycles reflect in our view two main geodynamic events that affected the southern europe after the variscan orogenesis the late carboniferous early permian transformation of the gondwana eurasia collisional margin into a diffuse dextral transform margin and the middle late permian opening of the neotethys ocean with the onset of a generalised extensional tectonic regime and the progressive westward marine ingression

a 420 000 year record of cyclicity in oceanic and atmospheric processes from the eastern equatorial pacific samples from the upper portion of a cyclic pelagic carbonate sediment sequence in deep sea drilling project dsdp hole 503b 4 0 n 95 6 w are the first group to be analyzed for paleoceanographic and paleoclimatic proxy indicators of ice volume deep ocean and surface water circulation and atmospheric circulation in order to resolve the complex origin of the cyclicity temporal resolution is taken from the 18o time scale most other parameters are calculated in terms of their mass flux to the seafloor caco3 percent in the sediments fluctuates in the well known pacific pattern and is higher during glacial times the fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2 5 to 4 the longer organic carbon record shows flux maxima during both glacial and interglacial times the accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods eolian dust fluxes are greater during interglacial periods by factors of 2 to 5 indicating that eolian source regions in central and northern south america were more arid during interglacial periods the record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds the eolian data suggest more intense atmospheric circulation during interglacial periods opposite to the anticipated results we interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503b during glaciations

a spectral analysis of geoid undulation and gravity anomaly data computed with pratt s isostasy theory applied to moho depth variations in fennoscandia summary the analysis shows that the spectra of geoid undulation n and gravity anomaly g computed from the values implied by the osu89b harmonic coefficients degrees 4 100 rapp pavlis 1990 do not fit the spectra of n and g computed from values implied by a pratt type isostatic model applied to the moho depth variations in fennoscandia as suggested by anderson 1984 and marquart 1989 the magnitudes of the model spectra are that much larger than the osu89b spectra that the relevance of the isostasy model is questionable

ages and compositions of primary and secondary allanite from the lala fe cu deposit sw china implications for multiple episodes of hydrothermal events numerous fe cu deposits in southwestern china form the kangdian iron oxide copper gold iocg metallogenic province these deposits have a close association of fe oxides and cu sulfides formed at different stages which are possibly related to multiple hydrothermal events in this paper u pb dating and chemical analyses on allanite from different stages of the lala deposit were used to constrain timing and origin of such events allanite occurs as disseminated grains or patches in fe cu ores and is closely associated with chalcopyrite molybdenite calcite and minor titanite postdating magnetite and apatite high resolution backscattered electronic bse imaging electron microprobe compositions and x ray scanning profiles demonstrate that ree rich primary allanite was replaced by later relatively porous and ree poor secondary allanite such a replacement was promoted by interaction between primary allanite and fluid fluxes infiltrating the minerals following an exchange scheme of ree3 fe2 ca2 al3 the secondary allanite has higher fe3 fe3 fe2 ratios and u contents indicating involvement of relatively oxidized fluids during alteration the alteration has also produced unidentified secondary ree minerals in fractures indicating re deposition of some of the removed rees the primary and secondary allanites are dated by in situ la icp ms technique and have u pb ages of 1 067 41 ma and 880 850 ma respectively the 1 07 ga primary allanite was contemporaneous with the main mo cu lree mineralization with a molybdenite re os age of 1 08 ga the 880 850 ma secondary allanite is comparable with the ar ar ages 890 830 ma of biotite from hosting schists and undeformed sulfide veins occurring throughout the kangdian province suggesting that such an event was possibly syn deformational and represents a younger hydrothermal event occurrences of both primary and secondary allanites suggest that the mineralization may have involved multiple tectonothermal events including the 1 05 1 1 ga intra plate and subsequent 960 740 ma arc magmatism in the kangdian region

characterising the u th pb systematics of allanite by id and la icpms implications for geochronology abstract allanite has the potential to be a useful chronometer of crustal evolution forming in response to a wide spectrum of metamorphic and magmatic conditions and incorporating weight percent concentrations of lree th and u despite its growing use in in situ u th pb geochronology allanite reference materials lack sufficient u th pb isotopic characterisation and little is known concerning the response of u th pb systematics of allanite to hydrothermal alteration and self irradiation this contribution presents the results of a combined id tims and la icpms u th pb study on a suite of five allanite crystals spanning 2 6 ga and including three commonly used allanite reference materials the siss bona and tara allanites siss and bona allanites preserve an inherited ca 1 ga pb component consistent with the presence of xenocrystic allanite cores or the presence of zircon micro inclusions tara allanite yields concordant u pb ages 407 430 ma but is affected by th u fractionation likely caused by hydrothermal alteration additionally the tendency for th to become mobilised post crystallisation is further evidenced by two precambrian allanite megacrysts le40010 ca 2 8 ga and le2808 ca 1 1 ga that both exhibit discordant th pb analyses linked to the formation of thorite micro inclusions along hydration pathways self irradiation dose versus discordance relationships show that a percolation threshold is present in allanite at cumulative dose values close to 3 10 17 decay g 1 an order of magnitude smaller than zircon collectively the presence of common pb and excess 206 pb its susceptibility to incur th u fractionation and hydrothermal pb loss complicates the use of allanite as a geochronometer these factors explain dispersion of 4 in the isotopic compositions of siss and tara allanites measured by la icpms providing a fundamental limit on the accuracy of the allanite chronometer using these reference materials

seafloor geomorphology and glacimarine sedimentation associated with fast flowing ice sheet outlet glaciers in disko bay west greenland fast flowing outlet glaciers currently drain the greenland ice sheet gis delivering ice meltwater and debris to the fjords around greenland although such glaciers strongly affect the ice sheet s mass balance their glacimarine processes and associated products are still poorly understood this study provides a detailed analysis of lithological and geophysical data from disko bay and the vaigat strait in central west greenland disko bay is strongly influenced by jakobshavn isbrae greenland s fastest flowing glacier which currently drains 7 of the ice sheet streamlined glacial landforms record the former flow of an expanded jakobshavn isbrae and adjacent gis outlets through disko bay and the vaigat strait towards the continental shelf thirteen vibrocores contain a complex set of lithofacies including diamict stratified mud interbedded mud and sand and bioturbated mud deposited by 1 suspension settling from meltwater plumes and the water column 2 sediment gravity flows and 3 iceberg rafting and ploughing the importance of meltwater related processes to glacimarine sedimentation in west greenland fjords and bays is emphasised by the abundance of mud preserved in the cores radiocarbon dates constrain the position of the ice margin during deglaciation and suggest that jakobshavn isbrae had retreated into central disko bay before 10 6 cal ka bp and to beyond isfjeldsbanken by 7 6 7 1 cal ka bp sediment accumulation rates were up to 1 7 cm a 1 for ice proximal glacimarine mud and 0 007 0 05 cm a 1 for overlying distal sediments in addition to elucidating the deglacial retreat history of jakobshavn isbrae our findings show that the glacimarine sedimentary processes in west greenland are similar to those in east greenland and that variability in such processes is more a function of time and glacier proximity than of geographic location and associated climatic regime

carbonate dissolution in recent sediments of the eastern equatorial indian ocean preservation patterns and carbonate loss above the lysocline abstract multiple dissolution indices from 45 sediment samples show that the foraminiferal lysocline is near 3800 m in the eastern equatorial indian ocean close to the saturation horizon with respect to calcite in the water column below the lysocline the increase in foraminiferal fragmentation decrease in absolute abundance and progressive modification of planktic foraminiferal assemblages with depth is accompanied by an increasing rate of loss of carbonate from the sediments above the lysocline some dissolution must occur in order to produce the dramatic reduction in foraminiferal numbers up to 60 and the pattern of increased fragmentation with depth however the amount of carbonate actually lost from supralysoclinal sediments reaches at most 20 30 of the total at the depth of the lysocline the principal driving force behind supralysoclinal dissolution is most likely the oxidation of organic matter incorporated into the sediments dissolution above the lysocline has a greater apparent impact on relative preservation in the coarse 150 m fraction than it does on total carbonate loss because of redistribution of carbonate to the finer size fractions as fragments though the effects of dissolution on foraminiferal preservation are clearly visible at all depths the lysocline itself does appear to mark the local onset of large scale loss of dissolved carbonate from the sediments such an interpretation is broadly consistent with models regarding the saturation state of the overlying waters as the principal driving force behind the preservation or dissolution of recent carbonate sediments on the sea floor

regular and irregular climatic cycles and the belemnite marls pliensbachian lower jurassic wessex basin time series of carbonate and organic carbon values are reported for 10 5 m of the belemnite marls the light and dark marl beds form decimetre scale couplets and metre scale bundles light marls are carbon poor and carbonate rich whereas dark marls and laminated shales are carbon rich and carbonate poor sediment composition appears to have been indirectly controlled by climate the regular couplets may record orbital precession cycles 20 ka however the bundles cannot be linked to changes in eccentricity rather they apparently record irregular large amplitude climatic variations with periods of a few hundred thousand years it should not be assumed that 100 400 ka cyclicity in climate related sedimentary sequences can be explained only by orbital forcing

dating iron skarn mineralization using hydrothermal allanite la u th pb isotopes by laser ablation icp ms abstract trace elements and u th pb isotopes of hydrothermal allanite from the beiminghe iron skarn deposit eastern north china craton were simultaneously analyzed using laser ablation inductively coupled plasma mass spectrometry la icpms to demonstrate the suitability and robustness of the u th pb system in this phase as a geochronometer of ore forming processes allanite grains from the beiminghe deposit typically exhibit optical and chemical zoning and are intergrown or texturally equilibrated with prograde and or retrograde skarn assemblages mainly consisting of tremolite diopside magnetite and garnet in addition they contain abundant aqueous two phase or daughter mineral bearing three phase aqueous fluid inclusions confirming their hydrothermal origin most grains have high la ce ratios 1 19 1 74 and thus are classified as allanite la a notable feature of the hydrothermal allanite la is the lack of common pb with values comparable with those of the zircon standards 91500 and gj 1 used in this study la icpms spot analyses reveal remarkable variation in ca rees rare earth elements fe al and mg within a single grain consistent with the substitution of ree3 mg fe2 for ca al fe3 in the mineral structure traverses of zoned grains show that the rims have th u ratios ranging from 0 73 to 11 55 significantly higher than the core areas 0 07 1 81 fifty nine spot analyses beam size of 32 m or 60 m on 5 allanite la grains yield reproducible 206pb 238u ages of 134 7 to 139 6 ma with a weighted mean 206pb 238u age 206pbexcess corrected 206pb 238u age of 136 1 ma 2 of these 34 analyses on allanite rims have a weighted mean 208pb 232th age of 139 2 ma 2 that is within error of the 206pb 238u age the dating results showed that an increase of beam size from 32 to 60 m significantly reduces the analytical uncertainties 2 petrographic and textural data have demonstrated that the allanite la grains formed during skarn alteration and notably have close paragenetic relationships with retrograde skarn assemblages and iron oxide minerals their u th pb ages therefore are interpreted as the timing of iron skarn mineralization at beiminghe thirteen zircon grains from the ore related diorite intrusion have a weighted mean 206pb 238u age of 136 2 ma 2 consistent with and thus confirming the reliability of the allanite la u th pb ages when interpreted as the formation age of the beiminghe iron deposit our results suggest that hydrothermal allanite with low th and u is a promising u th pb geochronometer and can provide reliable constraints on the timing of hydrothermal mineralization without requiring a matrix matched external standard the present dating results also indicate that the u pb rather than th pb isotope system is preferable for u th pb dating of low th u allanite

magma pulses in the central variscan belt episodic melt generation and emplacement during lithospheric thinning magmatism in the central part of the variscan orogenic belt in europe has occurred in several distinct episodes each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting i between 340 and 330 ma visean high k diorites and granitoids within a zone extending parallel the orogen axis possibly tracing a palaeosuture ii at 310 307 ma westphalian calc alkaline granitoids in the intra alpine domain iii at 304 295 ma stephanian subalkaline granitoids in intra alpine and pennine domains and finally iv at 280 270 ma permian granitoids in pennine austroalpine and south alpine domains the generation and emplacement of granitoid melts in the variscan crust are suggested to be short term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle and decompressional melting

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

oligocene paleoceanography of the south atlantic paleoclimatic implications of sediment accumulation rates and magnetic susceptibility measurements paleoceanographic changes during the oligocene in the south atlantic have been examined using paleomagnetic rock magnetic carbonate content and sediment accumulation rate sar data we show that magnetic susceptibility ms is largely controlled by carbonate content at deep sea drilling project site 522 which in turn is the primary control on sar at the site over 5000 ms measurements were averaged over magnetostratigraphic polarity intervals for use as indicators of carbonate content and sar proxies the study produced three primary results 1 a spectral analysis of the ms data from one upper oligocene core reveals two dominant periodicities 44 7 10 4 kyr and 26 4 3 6 kyr which approximate milankovitch obliquity 41 kyr and precession 23 kyr periods 2 four pulses of increased sar lasting 1 to 2 m y punctuate the section implying longer term climatic changes these events are also seen in the contessa quarry section of the italian tethyan oligocene and 3 two of the sar maxima correspond to times of global cooling and possible ice volume increases at 31 and 36 ma

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

thpb versus upb isotope systematics in allanite from co genetic rhyolite and granodiorite implications for geochronology uthpb isotopic analyses were performed on single allanite crystals from a rhyolite of the atesina volcanic complex avc and a granodiorite of the cima d asta pluton cap which form the largest calc alkaline volcano plutonic association of late hercynian age in the south alpine domain northern italy the best approximation to the time of emplacement of the avc and cap is given by essentially coeval 232th208pb ages of 276 3 2 2 ma 95 confidence limit n 2 and 275 5 1 5 ma n 4 for avc rhyolite and cap granodiorite respectively although the avc cap magmatites have been overprinted by a triassic hydrothermal event the thpb isotopic system in allanite remained virtually undisturbed whereas the upb systems display variable degrees of open system behaviour the allanite thpb ages are neither affected by initial radioactive disequilibrium nor by the inheritance of radiogenic pb components which underlines the importance of the thpb isotopic system for accurate dating of th enriched minerals the isotopic results demonstrate that even hydrothermally altered magmatites can successfully be dated by this technique

206pb 230th 234u 238u and 207pb 235u geochronology of quaternary opal yucca mountain nevada abstract u th pb isotopic systems have been studied in submillimeter thick outermost layers of quaternary opal occurring in calcite silica fracture and cavity coatings within tertiary tuffs at yucca mountain nevada usa these coatings preserve a record of paleohydrologic conditions at this site which is being evaluated as a potential high level nuclear waste repository the opal precipitated from groundwater is variably enriched in 234u measured 234u 238u activity ratio 1 124 6 179 and has high u 30 313 ppm low th 0 008 3 7 ppm and low common pb concentrations measured 206pb 204pb up to 11 370 it has been demonstrated that the laboratory acid treatment used in this study to clean sample surfaces and to remove adherent calcite did not disturb u th pb isotopic systems in opal the opal ages calculated from 206pb 238u and 207pb 235u ratios display strong reverse discordance because of excess radiogenic 206pb derived from the elevated initial 234u the data are best interpreted using projections of a new four dimensional concordia diagram defined by 206pb 238u 207pb 235u 234u 238uactivity and 230th 238uactivity ages and initial 234u 238u activity ratios have been calculated using different projections of this diagram and tested for concordance the data are discordant that is observed 207pb 235u ages of 170 32 2 to 1772 40 ka are systematically older than 230th u ages of 34 1 0 6 to 452 32 ka the age discordance is not a result of migration of uranium and its decay products under the open system conditions but a consequence of noninstantaneous growth of opal combined u pb and 230th u ages support the model of slow mineral deposition at the rates of millimeters per million years resulting in layering on a scale too fine for mechanical sampling in this case u pb ages provide more accurate estimates of the average age for mixed multiage samples than 230th u ages because ages based on shorter lived isotopes are nonlinearly biased by younger mineral additions use of the combined u th pb technique to date yucca mountain quaternary opals significantly extends the age range beyond that of the 230th u dating method and shows that selected fracture pathways in the unsaturated zone felsic tuffs of yucca mountain have been active throughout the quaternary

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

allanite u th pb geochronology by laser ablation icpms abstract the rare earth element ree rich epidote group mineral allanite has great potential as a geochronometer for a wide range of geological processes its utilization has been hampered by several analytical challenges often high common pb contents 208pbcomon 208pbtotal from using a 213 nm nd yag laser coupled with an agilent 7500cs quadrupole icpms analytical protocols focused upon minimizing the causes of matrix sensitivity in the laser ablation icpms process this has primarily been achieved via dynamic raster ablation which greatly reduces time dependent laser induced elemental fractionation accordingly a non matrix matched external standardization approach is adopted utilizing the zircon standards plesovice 91500 and gj1 accurate common pb corrections are critical to allanite geochronology and here we advocate an approach based upon the measured intensity of 204pb as it minimizes assumptions and allows for simple and robust error propagation for each of the allanite reference materials mean common pb corrected 208pb 232th ages and tera wasserburg regression ages are within uncertainty of reference values three of the samples siss bona and avc have significantly higher 206pb 238u ages which reflects incorporation of 230th during crystallization and hence 206pb excess such isotopic disequilibrium together with the high th u ratios of allanites up to 1000 makes the th pb dating system preferable particularly for relatively young allanites i e phanerozoic that accurate age information has been generated from allanites of wide ranging composition suggests that matrix effects are not significant in our analyses on the scale of uncertainties generated 0 5 to 1 5 2 on mean 208pb 232th ages the final propagated uncertainty in ages is a function of the common pb content uncertainty in the measurement of 202hg 204pb and uncertainty in the isotopic composition of the common pb inherited into the mineral our results show that accurate u th pb geochronological information at geologically useful levels of precision can be determined from allanite with relatively simple analytical and data reduction protocols and without the requirement for matrix matched standardization

paleoenvironmental implications of rock magnetic properties of late quaternary sediment cores from the eastern equatorial atlantic rapid and nondestructive rock magnetic measurements of late quaternary sediment cores from the senegal continental slope and the sierra leone rise in the eastern equatorial atlantic correlate strongly with the specmap oxygen isotope master curve and carbonate content the rock magnetic paleoclimatic relationship results from differences in the concentration accumulation rate grain size and composition of magnetic minerals between glacials and interglacials at both sites magnetic mineral assemblages of glacial episodes are characterized by higher concentrations and accumulation rates of magnetic minerals by coarser grained ferrimagnetic minerals e g magnetite and by a lower ratio of ferrimagnetic to antiferromagnetic e g goethite hematite minerals by serving as a sensitive tracer for various deep sea sedimentary components rock magnetic properties can provide insights into changing sediment sources and fluxes and therefore are a useful addition to more traditional chemically and mineralogically based paleoceanographic techniques

redistribution of rare earth elements thorium and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism the role of apatite and monazite in orthogneisses from southwestern norway the amphibolite to granulite facies transition has been studied in a high k calc alkaline hornblende biotite k feldspar megacrystic augen gneiss series from the rogaland vest agder rog va sector of the sveconorwegian province southwest norway hornblende begins to break down mainly to clinopyroxene at the cpx in isograd and biotite mainly to orthopyroxene at the opx in isograd abbreviations for minerals following kretz 1983 the magmatic accessory mineral association comprises titanite and allanite which begin to break down before the cpx in isograd titanite is preserved as relict inclusions in other minerals at higher grade monazite and thorite formed in the breakdown of allanite monazite abundance reaches a maximum between the cpx in and opx in isograds the middle and heavy rare earth element m hrees except eu content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite hornblende and biotite the u contents of apatite are low and do not increase with metamorphic grade the light rare earth elements lrees and th content of apatite are not correlated to the breakdown of allanite around the cpx in isograd but increase around the opx in isograd a simplified equation is proposed for monazite crystallisation in the vicinity of the cpx in isograd that accounts for the allanite titanite and hornblende breakdowns and the m hree substitution in apatite 3 m hree 2o3in hornblende and litanite 3 lree 2in allaniteo3 2ca5 po4 3apatite f oh 6sio2quartz 6 lree pmonaziteo4 2ca2 m hree 3 sio4 3 f oh lessingite in apatite 6caoin plagioclase at the opx in isograd the increase of lrees and th content of apatite results either from the breakdown of some monazite 3 lree po4monazite 3sio2quartz 4caoin plagioclase f2 h2o fluid ca5 po4 3 f oh apatiteca2 lree 3 sio4 3 f oh less ingite in apatite or the breakdown of the remaining allanite 3 lree 2o3in allanite 6sio2quartz 4caoin plagioclase f2 h2o fluid 2ca2 lree 3 sio4 3 f oh lessingite in apatite the release of fluorine from the breakdown of biotite at the opx in isograd may increase apatite stability relatively to monazite in the granulite facies when compared to amphibolite facies the granulite facies augen gneisses do not show any u or th depletion this means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution the coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that high k calc alkaline orthogneisses are a suitable material to date with the u pb method prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

signal analysis of cyclicity in maastrichtian pelagic chalks from the danish north sea abstract low field bulk magnetic susceptibility has been determined on maastrichtian chalk samples from a drill core from the dan field in the danish north sea fast fourier transformations fft have been used to detect possible cycles in the magnetic susceptibility data power spectra from the complete section and from sub sections of the magnetic susceptibility reveal two cyclicities of ca 0 4 cycles m and ca 1 7 cycles m which are present on a 90 confidence level signal analysis of natural gamma ray wire line log data supports these findings sedimentation rate estimates place the cycles in the milankovitch frequency band artificial time series are used to study the applicability of the fft to identifying cyclicity in chalks expected geological distortions e g hiati and sedimentation rate variations are introduced into the time series to investigate the response of the frequency spectra different methods of handling missing data intervals are also examined

early diagenesis in an organic rich turbidite and pelagic clay sequence from the cape verde abyssal plain ne atlantic magnetic and geochemical signals abstract processes of early diagenesis in sediments from the cape verde abyssal plain are investigated using the complimentary techniques of environmental magnetism and solid phase geochemistry the sequence captured by a 2 m long box core comprises an organic rich turbidite derived from the nw african continental margin emplaced ca 330 ka bp overlain by brown pelagic clay diagenesis in the pelagic clay mainly involves slow in situ oxidation of the iron oxide phases present which mossbauer effect data suggest are detrital haematite and magnetite possibly maghaemitised in the turbidite diagenesis has proceeded in two stages the first involving suboxic reductive processes following depletion of porewater o2 and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of o2 from the overlying seawater carbonate free magnetic parameters interparametric quotients and remanence acquisition demagnetisation and coercivity measurements of the core reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains like titano magnetite has occurred within the turbidite during suboxic diagenesis and confirm previous suggestions that reductive dissolution of such components is a grain size selective process however the higher oxidation state fe3 canted antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic fe mn micronodules in a distinct layer immediately overlying the front and above this a further interval of mn enriched diagenetic laminae however within the suboxic organic rich zone of the turbidite well below the oxidation front there is clear evidence for the presence of a significant concentration of fine single domain ferrimagnetic iron oxide grains which are most probably associated with a population of live magnetotactic bacteria between this zone and the oxidation front there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly and fossil magnetosomes in the sediment are being reductively dissolved progressively up sequence these results are broadly similar though differing in some respects to magnetic and geochemical studies of hemipelagic and high productivity pelagic sediments in which mn and fe redox boundaries have been recognised and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

rock magnetic characterization of early redoxomorphic diagenesis in turbiditic sediments from the madeira abyssal plain rock magnetic measurements of two sediment cores from the madeira abyssal plain map north atlantic are used to investigate post depositional changes in the concentration grain size and composition of magnetic minerals in the sediments that have occurred within organic rich turbidite horizons the changes are associated with an initial stage of suboxic reductive diagenesis following depletion of porewater o2 and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment as a result of diffusion of o2 from the overlying sea water the turbidites are of late quaternary age 18o stages 1 3 and derive both from different sites on the nw african continental margin and from the flanks of the canary islands thus the turbidites are variable compositionally especially in terms of carbonate detrital magnetic mineral and organic carbon content diagenetic changes in these sediments have been identified using solid phase geochemical data u mn corg and caco3 reported previously in more than one study rock magnetic parameters of the sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within organic rich turbidites during redoxomorphic diagenesis normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively and bulk magnetic concentration is thus depleted there is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic rich turbidites beneath both active and fossil oxidation fronts these grains are most probably associated with populations of live magnetotactic bacteria which commonly inhabit such organic rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results show that simple and rapid rock magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep sea sediments at least as effectively as more laborious time consuming and sample destructive geochemical measurements

evidence for a change in the periodicity of tropical climate cycles at 2 4 myr from whole core magnetic susceptibility measurements the earth s orbit about the sun is influenced by gravitational interactions with the moon and other planets the resulting orbital perturbations give rise to cyclical variations in orbital eccentricity obliquity and precession with periods of 100 41 19 and 23 kyr respectively1 these variations are climatically important because they affect the seasonal and latitudinal distribution of solar radiation magnetic susceptibility measurements in deep sea sediment cores can be used as a sensitive indicator of temporal variations in the concentration of terrigenous material supplied to the sea bed here we use this approach to study the evolution of cycles of terrigenous sedimentation at ocean drilling program sites in the arabian sea and the eastern tropical atlantic we show from spectral analyses of the two susceptibility time series spanning the past 3 2 3 5 myr that the records are driven strongly by orbital forcing before 2 4 myr before present bp both records are strongly influenced by the 23 and 19 kyr periodicities but after 2 4 myr they are both significantly influenced by the 41 kyr periodicity this shift coincides with the beginning of major northern hemisphere glaciation which indicates for the indian ocean site that the supply of terrigenous sediment carried by monsoonal winds has responded to the rapid increase in ice cover at this time we cannot however exclude possible effects of changes in bottom water circulation on the susceptibility record of the atlantic site

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

permian metamorphic event in the alps in addition to the well known variscan and alpine orogenic cycles a permian thermal event occurred in the european alps the geodynamic background of this event is not well understood here we suggest that the event is a reflection of lithospheric thinning accompanied by magmatic underplating causing partial melting of the lower crust and low pressure high temperature metamorphism the event was terminated by early triassic opening of the meliata ocean and was followed by sag stage subsidence during slow lithospheric cooling this last stage of the evolution allowed the onset of the mesozoic marine evolution with the orogen wide formation of triassic carbonate platforms well known from the dolomites and the northern calcareous alps the combined evidence for magmatism metamorphism and late subsidence identifies the permian event of the alps as an excellent example of underplating events suggested as a cause for low pressure high temperature terrains worldwide

mineral magnetic and textural interpretations of sedimentation in the skagerrak eastern north sea abstract the mineral magnetic properties of surface sediments in the northern skagerrak show systematic trends which allow the identification of sediment provinces and the interpretation of transport pathways this application is especially valuable considering the importance and difficulty of environmental analyses involving fine grained deposits magnetic susceptibility and saturation isothermal remanent magnetisation sirm have different geographic patterns but their individual trends are consistent in the skagerrak muds clay content 25 60 over the entire studied area the values correlate positively with general trends in particle size whereas sirm does not show particle size dependency the present study confirms that the majority of sediments accumulating in the northern skagerrak are supplied from the south the mineral magnetic parameters facilitate detailed tracking of the counterclockwise transport of sediments around and into the norwegian trench

milankovitch cycles in neocene deep sea sediment pelagic carbonate sediments from the world ocean basins commonly show cyclic variations in amount and or degree of preservation of biogenic calcite with periodicities of several tens to several hundreds of thousands of years the direct causes of these cycles are fluctuations in noncarbonate dilution carbonate production carbonate dissolution and or current winnowing the overall driving force however is variation in the earth s orbital characteristics milankovitch cycles through their influence on global climate and depositional processes the main inferred climatic effects of orbital perturbations are on global ice volume global temperature ocean circulation and distribution of climatic patterns eustatic sea level is directly related to ice volume as is the rate of erosion of clastic material from continental margins changes in volume of sea ice affect the volume and intensity of bottom water flow which in turn may cause changes in the intensity of sediment winnowing by bottom currents in the intensity of upwelling of nutrient rich bottom waters and in the depth of carbonate dissolution change in productivity of calcareous plankton are difficult to prove as a cause of carbonate cycles but may have contributed to the formation of carbonate cycles off northwest and southwest africa fluctuations in winnowing of fine grained components have been demonstrated as a cause of cyclic variations in the coarse fraction component of carbonate sediments in the southwest pacific dilution of carbonate by clastic material probably was a major cause of fluctuations in carbonate content of deep sea sediments off northwest and southwest africa carbonate dissolution cycles probably are the most common manifestation of fluctuations in bottom water flow dissolution cycles are common in quaternary and neogene sediments of the north atlantic caribbean and eastern equatorial pacific the main cause of the carbonate dissolution was shoaling of the carbonate compensation depth during the early neogene in response to climatically induced fluctuations in the thickness of antarctic bottom water

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

isostatic topography and crustal depth corrections for the fennoscandian geoid abstract the medium wavelength geoid wavelengths the result of this study demonstrate that crustal thickness variations are important for the interpretation of the fennoscandian geoid or gravity however submoho deviatoric stresses might become very high about 200 mpa if the entire anomaly is assumed to be supported by lateral density variations at less than 80 km depth this implies that the pronounced negative anomaly is additionally maintained by dynamical flow inside the mantle

allanite behaviour during incipient melting in the southern central alps abstract the response of allanite to incipient melting was investigated in migmatites from the tertiary barrovian type sequence of the central alps southern switzerland northern italy inheritance and new mineral growth were recorded in composite allanite grains sampled from meta granitoids and leucosomes ion microprobe shrimp dating of high th u allanite cores in meta granitoids yield permian ages consistent with magmatic crystallisation dating protolith intrusion in contrast low th u allanite overgrowths and weakly zoned allanite in meta granitoids and leucosomes yield alpine u pb intercept ages between 30 4 and 20 5 ma these date allanite formation during the barrovian cycle major and accessory mineral ree compositions suggest that alpine allanite crystallised in the presence of a low temperature melt whereas new zircon growth is rare in the migmatites allanite readily recorded growth during the alpine cycle allanite u th pb isotopes may therefore present a complementary approach to zircon for dating low temperature partial melting where the preservation of allanite is aided by low lree solubility in hydrous granitic melt the th pb age is preferred to date high th magmatic allanite however the u pb and th pb ages of allanite overgrowths may differ by up to 25 and this demands a comparison of both u pb and th pb isotopic systems to obtain a best estimate for the timing of low th allanite crystallisation protolith allanite preserves a substantial memory of its initial age in spite of upper amphibolite facies re working during migmatisation t 620 700 c which places strong constraints on pb closure temperature magmatic allanite contains 60 of initial pb therefore the initial pb may be useful as a sensor for the amount of melt present during allanite formation the pb isotope composition of allanite overgrowths indicates 5 inherited radiogenic pb from precursor allanite which suggests efficient redistribution and homogenisation of pb isotopes during the alpine partial melting period

orbital forcing of bottom current enhanced sedimentation on feni drift ne atlantic during the mid pleistocene feni ridge is a plastered sediment drift deposited along the northwestern flank of rockall trough ne atlantic under the influence of deep geostrophic currents formed by intermittent overflows of arctic intermediate water from the norwegian sea across the iceland scotland ridge nsow lower deep water ldw a derivative of antarctic bottom water from the south atlantic mixed with old north atlantic deep water nadw also bathes the lower flanks of the drift below 2500 m deep sea drilling project site 610 was drilled in a sediment wave field on the surface of feni drift 2417 m though the sediments recovered proved to be typically pelagic preserving a high resolution 5 cm kyr 1 quaternary record of orbitally modulated ice rafting and biogenic productivity cycles but showing little evidence of current affected deposition from particle size and compositional analyses of the biogenic and lithogenic 10 m silt which we then compared with records of the percentage and flux of ice rafted detrital ird sand foraminifera and coccoliths for the same interval cross spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume ice sheet growth and decay in response to orbitally forced insolation changes at 65 n variations in the relative proportions and fluxes of ird sand foraminifera and coccoliths all covary closely with orbitally forced ice volume fluctuations but with differing amplitude coherency and phase relationships at each milankovitch frequency however there is a significant component of variance in pelagic and ice rafted sedimentation which is controlled by bottom currents and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies the relative enrichment coarseness and flux of the contourite silt fraction of feni drift sediments all increase during glacial or cold stages of the mid pleistocene relative to warm stages suggesting that currents were stronger during these periods possible explanations for such changes in current activity include 1 an increased flux of nsow via the wyville thompson ridge into the rockall trough during glacials but not due to increased production of nsow 2 ldw presently lying below 2500 m rises during glacials by at least 50 60 m thereby flooding over the surface of feni ridge at site 610 3 the path of a major recirculation gyre in the lower nadw ldw system presently situated marginally south of site 610 encroaches upon the southern tip of feni ridge during glacials and 4 the rockall trough itself is a site of deep water formation during glacials analogous to labrador sea deep water formation today

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

petrochronology based on ree minerals monazite allanite xenotime apatite ree minerals monazite xenotime and allanite are ree1 minerals sensu stricto because lanthanides la lu and yttrium are critical constituents in them apatite does not require ree but because it contains substantial ree in many rocks it is included in this review all four minerals also host unusually high radionuclide concentrations notably th and u forming the basis of their utility as geochronometers this quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic p t conditions so can lend petrogenetic context to chronology based on other minerals part i of this review assembles the basic requisites prior to integrative petrochronologic analysis individual characteristics of the four ree minerals are addressed first i e their crystal chemistry and stability relations thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next and finally their chronology is summarized part ii presents case studies to highlight the specific strengths of ree minerals used to resolve the dynamics of a broad range of processes from diagenetic to magmatic conditions finally a brief section at the end outlines a few of the current challenges and promising perspectives for future work to introduce the four ree minerals in style let us recall the origins of their names the three phosphates have well respected greek grandparents and allanite has solid scottish roots yet of all four of them show idiosyncracies in etymology or type material apatite had long puzzled naturalists as it shows great chemical and physical variability and can resemble other minerals once properly identified abraham gottlieb werner named it apatite his reasoning referred to the greek root and giving the precise latin translation decipio taken literally both mean i deceive or i mislead which sounds like an apt confession from this mineral for having fooled

submarine landforms and glacimarine sedimentary processes in lomfjorden east spitsbergen understanding the role of fjords in modulating the long term interaction between ice sheets and glaciers with the surrounding ocean requires the investigation of glacigenic landform and sediment archives in svalbard there is a wealth of data from fjords in west spitsbergen that constrains the glacial history of this sector of the svalbard barents sea ice sheet sbis since the last glacial maximum lgm and the nature and timing of subsequent ice retreat in contrast however very little is known about the glacial history of fjords in east spitsbergen this paper combines multibeam swath bathymetry sub bottom profiles lithological data and radiocarbon dates from lomfjorden svalbard to provide the first insights into the dynamics of tidewater glaciers and associated glacimarine sedimentary processes in a northeast spitsbergen fjord at the lgm a fast flowing ice stream drained the sbis through lomfjorden serving as a tributary to a south north flowing ice stream in hinlopenstretet ice advance is recorded by streamlined bedrock glacial lineations and drumlins a radiocarbon date of 9 7 ka bp from the outer fjord provides a minimum date for retreat of this ice stream and suggests that lomfjorden was ice free earlier than fjords in west spitsbergen ice retreat occurred in a slow and step wise manner indicated by the presence of recessional moraines and de geer moraines by 4 5 ka bp the local tidewater glaciers had probably retreated inland of their present positions the limited extent of glacigenic landform assemblages in front of these glaciers implies that any holocene re advances were probably restricted the principal sedimentary processes during deglaciation were suspension settling from meltwater causing deposition of weakly stratified bioturbated mud in ice distal settings at rates of 0 02 0 08 cm a 1 and gravitational mass flows forming sandy turbidites in ice proximal areas iceberg ploughmarks and ice rafted debris provide evidence for the presence of large icebergs during deglaciation our data suggest an early and extensive deglaciation in east spitsbergen fjords and show that previous reconstructions of the extent of the sbis need to be revised as new data emerges from east spitsbergen the data confirm that tidewater glaciers from different regions of spitsbergen behaved differently since the lgm and that variations in landform sediment assemblages occur even within geographically adjacent fjords

predicting radioactive accessory mineral dissolution during chemical weathering the radiation dose at the solubility threshold for epidote group detrital grains from the yangtze river delta china abstract the influence of heavy ion irradiation on the solubility of natural epidote group minerals has been investigated the epidote group are calc silicate minerals that if soluble during terrestrial chemical weathering are capable of consuming atmospheric co 2 over geologic time the experimental design was to calculate the particle dose for actinide rich epidote group grains from the deltas of rivers draining large watersheds large watersheds should yield a suite of epidote group grains spanning the range of compositions and radiation damage commonly observed in nature these grains would be resistant to dissolution during chemical weathering and survive into fluvial sediments therefore the highest particle dose calculated for the suite of grains represents a minimum solubility threshold for the epidote group minerals during chemical weathering the particle dose is calculated from the 232 th 238 u and 235 u content of a grain and the grain s date actinide rich epidote group grains were isolated from yangtze river and river nile delta sediments although only the grains from the yangtze delta had 232 th 208 pb ratios sufficiently high to yield meaningful dates a total of 28 th rich epidote group grains were isolated from the yangtze delta sediment with 24 being classified as allanite and four being classified as ree rich epidote isotopic ratio data were measured by laser ablation microprobe inductively coupled plasma mass spectrometry dates and ree patterns revealed at least 17 different sources of grains with tho 2 contents ranging from 0 057 to 4 37 wt and dates ranging from 32 2 to 3788 263 ma the calculated maximum particle dose of the suite of yangtze delta epidote group mineral grains and hence the minimum solubility threshold is 7 1 10 15 decay mg 1 therefore to predict the likelihood of an epidote group mineral dissolving during chemical weathering at a specific study site the particle dose may be calculated from the date of the rock even if estimated and the radioactinide concentrations of the mineral a calculated particle dose below 7 1 10 15 decay mg 1 likely reflects epidote group mineral stability during chemical weathering calculation of the particle dose may be superior to microscopic identification of accessory the particle dose at the solubility threshold of 7 1 10 15 decay mg 1 value is approximately double that reported for zircon therefore relative to zircon epidote group minerals can withstand more radiation prior to becoming soluble this finding is surprising considering the chemical durability of zircon this substantial variance in solubility threshold radiation doses between the two minerals cannot be explained by the bond strengths of non tetrahedral cations and structural oxygen which are lower in allanite than zircon

structural effects of the crust on the geoid modelled using deep seismic sounding interpretations summary according to the theory of isostasy the earth has a tendency to deform its surface in order to reach an equilibrium state the land uplift phenomenon in the area of the fennoscandian shield is thought to be a process of this kind the geoid as an equipotential surface of the earth s gravity field contains information on how much the earth s surface departs from the equilibrium state in order to study the isostatic process through geoidal undulations the structural evects of the crust on the geoid have to be investigated the structure of the crust of the fennoscandian shield has been extensively explored by means of deep seismic sounding dss the data obtained from dss are used to construct a 3 d seismic velocity structure model of the area s crust the velocity model is converted to a 3 d density model using the empirical relationship that holds between seismic velocities and crustal mass densities structural evects are then estimated from the 3 d density model the structural evects computed from the crustal model show that the mass deficiency of the crust in fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid it proves again that the crust has been isostatically compensated by the upper mantle in other words an anomalously high density upper mantle must exist beneath fennoscandia

evolution of large silicic magma systems new u pb zircon data on the nw permian athesian volcanic group southern alps italy abstract during the permian the southern alps were affected by prominent igneous activity that produced voluminous basic to acidic volcanic and plutonic rocks the magmatism originated during a period in which postorogenic extension and transtensional faulting controlled magma ascent and emplacement within this scenario the athesian volcanic group ag with lithologies ranging from basaltic andesites to rhyolites constitutes a major part of the permian magmatism in the central and eastern southern alps we present an up to date stratigraphic reconstruction of the northwestern sector of the ag volcanic sequence combining detailed field mapping and new geochronological data single zircon u pb age data suggest that the volcanism in the ag lasted ca 10 m yr between 285 and 274 ma with a deposition rate that increased throughout the eruptive cycle the processes of magma deposition and basin formation were strongly influenced by tectonic activity over the entire history of the ag at least two major

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

uranium lead system in fragments of a single zircon grain since the demonstration that u pb analyses of zircon fractions1 from a few grains up to several thousand grains or fragments of grains2 or single zircon grains3 4 yield reliable radiometric age data this method has been widely applied to igneous metamorphic and sedimentary rocks the present investigation of u pb systems for a suite of distinct fragments of a single zircon grain from a 1 093 2118 myr old syenite of the pikes peak batholith colorado usa demonstrates that uranium is heterogeneously distributed in this zircon crystal most parts of the grain suffered various degrees of lead loss the others show a closed system behaviour we show that degrees of lead loss are not correlated with uranium concentrations of the fragments

meso cenozoic basin and carbonate platform development in the sw dolomites unraveled by basin modelling and apatite ft analysis rosengarten and latemar northern italy abstract the rosengarten and latemar middle triassic carbonate platforms indicate similar subsidence patterns during equivalent times of platform evolution as calculated by numerical basin reverse modelling in the case of rosengarten and latemar thermo tectonic subsidence is the most significant among all three subsidence components thermo tectonic s flexural s compaction induced s hence the influence of differential tectonics as recorded in the formation of structural highs and lows in the dolomites during the lower to middle anisian almost ceases in late anisian times basin modelling of rosengarten and latemar requires the correlation and integration of two sets of chronostratigraphic data from buchenstein fm basin and schlern dolomite fm 1 platform subsidence modelling argues for a microcycle i e a single shallowing upward cycle duration of less than 1 97 ka in the cyclic succession at latemar however previous time series analyses identify milankovitch cyclicities in larger order cycle stacking patterns and thus indicate a microcycle duration of 4 2 ka according to each model total subsidence reaches values of 650 700 m ma 4 2 ka model or 780 850 m ma 1 97 ka model during the first stage of platform evolution as both platforms successfully keep up with this subsidence peak it is evident that the carbonate factory despite its low diversity must have recovered from the p t faunal crisis after this regionally observed peak subsidence drops and triggers fast progradation at rosengarten subsidence in the study area is controlled by active strike slip tectonics and the subsequent formation of a magmatic chamber predazzo monzoni above the deep reaching truden stava line the area close to this tectonic line and to the volcanic centre of predazzo monzoni undergoes a higher heat flow pulse during the late ladinian and is possibly buried at shallower levels than the rosengarten area as shown by fission tracks in apatites the heat flow pulse almost disappears laterally underlining the localised thermal influence of the late ladinian volcano thermal event of predazzo monzoni

dissolution reprecipitation of zircon at low temperature high pressure conditions lanzo massif italy abst r a c t an eclogite facies meta plagiogranite from the lanzo massif western alps italy contains crystals of zircon intimately associated with allanite zircon displays different microtextures ranging from pristine euhedral and magmatic to fractured porous varieties with mosaic zoning and pervasive recrystallization into euhedral microcrystals fractures and voids in the recrystallized zircon microcrystals are mainly filled by high pressure na rich pyroxene electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals with less than 2 misorientation among neighboring microdomains the textural change is coupled with chemical and isotopic modifications recrystallized zircon domains contain significantly less th and light to mid ree but are richer in sr than magmatic zircon crystals magmatic zircon preserves the protolith u pb age of 163 5 1 7 ma whereas zircon microcrystals have a mean age of 55 1 ma the coexisting allanite also contains inclusions of na rich pyroxene and has chemical features elevated sr and ni contents and lack of eu anomaly indicating formation at high pressure despite being associated texturally with zircon allanite yields a younger th pb age of 46 5 3 0 ma suggesting that the lanzo unit remained at relatively high pressure conditions for 8 m y zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution reprecipitation process recrystallization occurred with minimum transport in a low strain environment and was not significantly enhanced by metamictization the source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite

u th pb systematics of morphologically characterized zircon and allanite a high resolution isotopic study of the alpine rensen pluton northern italy abstract the rensen pluton is a typical member of the tertiary calc alkaline plutonic suite structurally connected to the periadriatic lineament it intrudes the austroalpine polymetamorphic basement of the eastern alps southwest of the tauern window northern italy zircons and allanites from tonalitic quartz dioritic and monzogranitic lithologies were studied systematically on the basis of refined grain by grain microanalytical techniques high resolution isotope analyses show that morphology and u pb systematics of zircon are highly correlated with magma evolution zircons from the main tonalitic part of the intrusion display a gradual increase of u content from early stage s type morphology to late stage g1 type morphology most of the zircon crystals include premagmatic seeds derived from old crustal sources unevolved s type zircons contain inherited components of late proterozoic early paleozoic apparent age whereas late stage g1 type zircons show inheritance of early proterozoic apparent age both types of inherited components are present in zircons from a fine grained tonalitic domain located at the center of the pluton which shows restitic structures the crustal affinity of this rock is corroborated by its relatively low e nd value 6 0 as compared to values obtained for the main tonalite 3 9 and the quartz diorite 3 5 the accuracy of cainozoic zircon ages is affected by initial radioactive disequilibrium in the u pb decay chains particularly uncertain is the fractionation behavior of protactinium which could be enriched in zircon due to the high stability of the pa 5 oxidation state because the th pb system is devoid of long lived intermediate radioactive nuclides the best approximation to the emplacement age of the rensen pluton is given by allanite th pb ages of 31 70 0 25ma 95 c l for the main tonalite and by a slightly younger age of 31 09 0 32ma for quartz diorite

u th pb and 230th 238u disequilibrium isotope systematics precise accessory mineral chronology and melt evolution tracing in the alpine bergell intrusion in order to investigate the potential of combined th u pb isotope and 230th 238u disequilibrium systematics for tracing magmatic crystallization and melt evolution conventional high resolution single crystal tims techniques have been applied to zircon titanite and fragments of geochemically characterized growth zones of allanite these minerals were extracted from a single tonalite specimen collected from the feeder zone of the tertiary bergell pluton southern steep belt s switzerland n italy the isotopic results document an extended history of crystallization and melt evolution of at least 5 ma with well resolved zircon ages defining an early interval of 33 0 to 32 0 ma followed by crystallization of zoned allanite from 32 0 to 28 0 ma and formation of magmatic epidote possibly as late as 26 ma trace and major element patterns in zoned allanite closely mirror melt evolution characterized by increase of u concentration and sharp decrease of th and lree reflecting early crystallization of phases low in u and in particular the dominating control by allanite precipitation preservation of substantial quantities of excess 206pb derived from initial excess 230th in all analyzed allanite grains indicates that their isotopic systems have not been reset by loss of radiogenic pb during prolonged residence at magmatic conditions and regional metamorphic cooling and that the measured sequence of 208pb 232th dates translates into a real age sequence major loss of radiogenic pb from compositionally zoned allanite by volume diffusion would have resulted in a data pattern grossly different from the observed one as demonstrated by numerical modeling of 232th 208pb 238u 230th 206pb isotopic evolution the results therefore suggest closure temperatures 700 c for magmatic allanite quantification of 230th 238u disequilibrium relationships reveals a smooth initially steep decrease of th u in the magma from values of 2 9 at 32 0 ma to 0 1 at 28 0 ma in equilibrium with sequential allanite zoning comparison of calculated th u magma with measured total rock th u 0 79 requires fractional crystallization of allanite at an early stage this process of removal of allanite and thus th from the melt was operative at least until 31 5 ma providing an upper limit for the time of emplacement of the studied magma batch the fact that bulk zircon and a large part of allanite crystallization predate emplacement cautions against equating ages determined on refractory minerals from deep seated plutons with intrusion ages the persistence of magmatic conditions over a period of 5 ma concentrated in a narrow collision zone between the european and the african plate has been a major controlling factor for the oligocene evolution of the adjacent insubric fault the main tectonic lineament of the alps being active from late cretaceous to miocene times

u pb zircon ages for post variscan volcanism in the ligurian alps northern italy abstract ages of permian volcanic rocks from the ligurian brianconnais domain western italian alps have been determined by laser ablation inductively coupled plasma mass spectrometry u pb dating of zircon three major volcanic units yielded zircons that were dated calc alkaline rhyolites 285 6 2 6 ma andesites with inherited cores yielded ages c 476 ma and older and voluminous rhyodacites rhyolites 272 7 2 2 ma following an amagmatic sediment starved time gap of c 14 ma alkaline volcanic activity is recorded at the top of the sequence by k alkaline rhyolites dated at 258 5 2 8 ma the ligurian segment of the southern variscan belt records transtensional and then extensional tectonics associated with the volcanic activity the switch from calc alkaline to alkaline activity corresponds to the transition from a post orogenic to an anorogenic setting in the southern variscides it may represent progressive and increasing delamination of the continental lithosphere accompanied by partial melting of the lithospheric mantle supplementary material technical and data acquisition parameters u th pb isotope analyses and calculated ages of zircons from samples and standard and trace element compositions of selected zircons are available at http www geolsoc org uk sup18322

constraints on fluid evolution during metamorphism from u th pb systematics in alpine hydrothermal monazite abstract the timing and duration of hydrothermal activity during orogenesis are difficult to constrain because such systems are open and multistage using mm sized monazite crystals from two alpine clefts from the central alps switzerland we demonstrate that combined u th pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization our data highlight four major characteristics of the cleft monazites i extreme th u ratios ii significant common pb in the u system up to 39 206 pb iii excess 206 pb up to 54 and iv precise and reliable th pb ages comparison of our results with literature data indicates that th u in monazite is a remarkable discriminant of geological conditions capable of distinguishing metasedimentary magmatic and hydrothermal origin hydrothermal monazite crystals are characterized by th u ratios up to 629 among the highest values ever reported extreme th u ratios in monazite enhance incorporation of 230 th a short lived intermediate product in the 238 u 206 pb decay chain generating 206 pb excess which if not accounted for results in 206 pb 238 u ages that are too old for example in the samples investigated here by as much as 300 for the two zoned monazite crystals studied in detail 208 pb 232 th ages are reliable for the different compositional domains visible in back scattered electron bse images 232 th 208 pb ages for the older domains 15 2 0 3 ma and 14 1 0 3 ma respectively are consistent with the structural relationships of the hydrothermal veins indicating early retrograde genesis the youngest rim age at 13 5 0 4 ma for the blauberg crystal marks termination of monazite precipitation the resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity in both crystals the outer rims have the highest 206 pb excess confirming a two stage crystallization two scenarios are envisaged to account for the 206 pb excess evolution in the first increasing 206 pb excess is caused by a modification of the th u fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent u into the fluid alternatively it is possible that 230 th increased with time in the fluid in this second case monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238 u 206 pb decay chain and progressively equilibrated t

the detection and illustration of regular sedimentary cycles using walsh power spectra and filtering with examples from the lias of switzerland power spectral analysis provides an objective method for the detection of regular cyclicity in data recording some oscillating parameter or time series digitization of measured stratigraphic sections is a convenient method for generating long time series during the examination of pelagic sequences for cyclicity related to orbital variations or milankovitch cycles the application of walsh power spectral analysis to stratigraphic sections is outlined and band pass filtering using the walsh transform introduced it is shown that variations in sedimentation rate hiatuses bioturbation and variations in compaction have the effect of damping or destroying spectral peaks the wavelength ratios of spectral peaks are often used during studies of milankovitch cycles to decide which orbital cycles are involved however in some circumstances hiatuses are able to alter wavelength ratios averaged walsh power spectra were generated for three liassic limestone shale sequences from breggia gorge switzerland the 21 000 year 21 ka precession cycle was detected in all three cases the 100 ka eccentricity cycle in two cases and the 41 ka obliquity cycle in one filtering revealed that the supposed 21 ka sedimentary cycles are grouped into packets of four or five small and large amplitude cycles the packets reflect the 100 ka modulation of the 21 ka precession cycle local disruption of the packaging can be used to locate hiatuses in pelagic strata

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

th pb ion microprobe dating of allanite allanite which is a common accessory mineral in a wide variety of rock types typically contains high concentrations of th and u thus an in situ method of u th pb dating of this phase would have broad application we describe a method to permit th pb ages of allanite to be determined with approximately 10 accuracy using a high resolution ion microprobe knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ion ization efficiencies of th and pb the chemical compositions of three allanite samples used as age standards cima d asta pluton 275 5 1 5 ma atesina volcanic complex 276 3 2 2 ma la posta pluton 94 2 ma were determined using an electron microprobe permitting an assessment of matrix effects on ionization an ion microprobe calibration curve involving elemental and oxide species of th and pb i e 208 pb th vs tho 2 th yields highly scattered apparent ages when allanite age standards with different fe contents are used however a three dimensional plot of 208 pb th vs tho 2 th vs feo sio improves the accuracy of the calibration to about 10 even though this level of uncertainty is substantially greater than that expected for u th pb ion microprobe analyses of zircon or monazite th pb ages of allanite can still be used to address important geologic questions we used this method to date two metamorphic allanite grains from the footwall of the main central thrust nepal himalaya and an allanite grain from the pacoima canyon pegmatite california allanite inclusions in garnet from nepal yield significantly older ages than the coexisting monazite indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault the pacoima canyon allanite grain yields a younger age than that reported for zircon implying pb loss during cooling of the pegmatite

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

allanite upb geochronology by 193 nm la icp ms using nist610 glass for external calibration abstract the viability of using nist610 glass as an external standard for allanite u th pb geochronology by 193 nm laser ablation inductively coupled plasma mass spectrometry was investigated laser ablation conditions were optimized to minimize time dependent fractionation of pb u and pb th a data reduction scheme vizualage umod was additionally used to apply pb u fractionation corrections when the standard contains an enriched 238u 235u ratio nist610 420 compared to natural materials optimization of ablation conditions was carried out on natural allanite material separated from a 415 ma tonalite intrusion hartfield intrusion new brunswick canada conditions of 25 s of ablations at 1 j cm2 3 hz and a 45 m crater diameter were established this produced 3 m deep craters and a near steady state low mass load material transfer to the icp ms producing negligible time dependent pb u and pb th fractionation increasing fluence only modestly up to 3 j cm2 induced time dependent pb u fractionation to test for any residual matrix dependencies the long term reproducibility accuracy and absolute precision of the method was verified using 31 5 ma siss3 allanite material bergell intrusion switzerland the method yielded accurate and precise pb u ad pb th ages that compare favourably to alternative allanite dating methods as a further test allanite separated from the hartfield tonalite was analyzed and yielded an age that overlaps within error of the tims titanite u pb age for the intrusion this is despite a partially amorphous microstructure as documented by raman spectroscopy and optical microscopy the results of this study emphasize the need to empirically calibrate laser fluence conditions that yield minimal element fractionation and to establish robust plasma settings these conditions are unlikely to yield highest sensitivity but should minimize the effects of the numerous laser induced and plasma ionization processes that lead to pb u fractionation

why allanite may swindle about its true age allanite from the tertiary rieserferner pluton austrian alps is texturally and chemically heterogeneous continuous covariation trends reflect coupled substitution of ca al vs fe ree th in allanite whereas systematic variations in la nd demonstrate the increasingly stronger depletion of lree in the melt during its crystallization allanite samples corrected for 206pbexcess from two rocks scatter in the concordia diagram and define discordias from 31 8 0 4 ma and 32 2 0 4 ma to ca 540 ma the apparent inheritance does not originate from the inclusion of older allanite or a high phase such as monazite xenotime or zircon but from the incorporation of radiogenic pb originating from a precursor since allanite requires a high enrichment of th u and lree it may form at the expense of a th lree rich precursor in metamorphic rocks or where such a phase had dissolved in melts likely precursors acquire with time radiogenic pb isotopic compositions this pb if incorporated in the product mineral may give the illusion of inheritance the allanite samples from the rieserferner pluton show a tendency from high th ucalc 30 50 in samples with an alpine age to low th ucalc 4 12 in samples with distinct inheritance this relation between extent of inheritance and apparent th ucalc indicates a lower th ucalc for a possible precursor falling into the th u range commonly encountered for monazite precursor monazite would have originated from assimilated palaeozoic rocks and give rise to localized enrichments of th and lree in the melt thus eventually enabling the growth of allanite

origin of allanite in gneiss and granite in the dabie orogenic belt central east china abstract allanite is a common accessory mineral phase representing an important carrier of rare earth elements th u sr and other trace elements in most continental rocks as th and u can be incorporated into the allanite lattice the mineral is a good geochronological tool for constraining geological events moreover the trace element features eu th u ratio and common lead content of allanite are indicators of the forming conditions allanite and coexisting epidote group minerals are abundant in ultrahigh pressure uhp metamorphic rocks from the dabie sulu orogen in central east china however if these minerals formed in the neoproterozoic as magmatic phases or in the triassic as metamorphic phases is a matter of long standing controversy we report major and trace element analyses of whole rocks allanite and coexisting epidote group minerals together with u th pb isotopic compositions of allanite in uhp gneiss from the dabie sulu orogen and allanite in the adjacent jingshan granite the granite is emplaced along the southeastern margin of the north china craton and considered a product of partial melting of the subducted dabie sulu gneiss trace elements low th u and la sm high eu and high sr and high common lead concentrations indicate a metamorphic origin of allanite epidote in the uhp gneiss on the other hand coarse grained allanite from the jingshan granite shows a corrosion core and a magmatic rim with common 208 pb up to 70 in the core and less than 30 in the rim the allanite cores are of peritectic and the rims of magmatic origin with ages of 160 ma consistent with the granite crystallization age in combination with previous studies we conclude that the allanite of the jingshan granite has form from the subducted and remolten dabie sulu gneiss allanite records triassic uhp metamorphic ages as well as jurassic peritectic magmatic ages as a part of the evolution of the dabie sulu orogen

conodont biostratigraphy and magnetic susceptibility of upper devonian chattanooga shale eastern united states evidence for episodic deposition and disconformities abstract recognition of stratigraphic hiatuses in fine grained siliciclastic sedimentary rocks can be challenging but is feasible using high resolution biostratigraphic and chemostratigraphic data within a regional correlation framework in this case study of the upper devonian chattanooga shale in the dupont ghs drillcore from the western margin of the nashville dome the upper devonian chattanooga shale comprises several depositional units separated by intraformational hiatuses these features are developed within a 13 3 m interval consisting of 4 2 m of frasnian strata and 9 1 m of famennian strata that unconformably overlie the sellersburg formation three frasnian seven famennian and one tournaisian conodont biozones are recognized the frasnian famennian boundary is on a disconformity to the northeast in the southern illinois basin the new albany shale in the bcc drillcore consists of 35 m of givetian and frasnian strata and 30 m of famennian strata that conformably overlie the sellersburg formation one givetian four frasnian and three of the four lowest famennian conodont zones are recognized the frasnian famennian boundary is conformable and constrained to within a 5 cm interval bulk magnetic susceptibility ms shows a long term increase through most of the frasnian shorter term ms trends were observed in association with depositional pulses linked to global sea level rises and highstand system tracts characterized by total organic carbon toc maxima eight trends were resolved in the dghs core and six in the bcc the high frequency shifts in ms likely represent milankovitch band sea level and depositional cycles at a scale that cannot be resolved based on the condensed and irregular nature of the depositional packages gamma ray counts in the dghs peak in association with the eight depositional cycles with a toc peak at the base of each cycle

stable and radiogenic isotope systematics in epidote group minerals epidote minerals the epidote group together with the orthorhombic polymorph zoisite occur in a wide range of igneous metamorphic and sedimentary lithologies although often present in only small quantities epidote group minerals can nonetheless be used to generate important quantitative constraints on processes such as regional metamorphism deep seated pluton emplacement and uplift hydrothermal fluid flow and sedimentary provenance e g zen and hammarstrom 1984 brandon et al 1996 cartwright et al 1996 keane and morrison 1997 spiegel et al 2002 petrologic studies on epidote group minerals formed by these and other processes are reviewed in bird and spieler 2004 grapes and hoskin 2004 enami et al 2004 and schmidt and poli 2004 in this chapter stable and radiogenic isotope studies of epidote group minerals will be reviewed although stable and or radiogenic isotope data exist for epidote group minerals from a number of different geologic settings such data have produced particularly important insights into 1 fluid flow associated with variable grades of metamorphism particularly ultra high pressure uhp metamorphism 2 intrusion of deep seated plutonic systems 3 the nature of hydrothermal alteration in geothermal systems 4 age relations in hydrothermal systems and 5 sedimentary provenance based on the chemical composition of naturally occurring epidote minerals oxygen hydrogen and chlorine stable isotopes all have the potential to be important petrogenetic indicators in systems involving epidote minerals oxygen is present in epidote minerals bound in isolated to4 tetrahedra t2o7 groups and in the hydroxyl site as both oh and o 2 hydrogen and chlorine are present in the hydroxyl site as oh and cl respectively although cl is present in only very small quantities frei et al 2004 to date oxygen and hydrogen isotope systematics have been studied both

late pleistocene paleoclimatology of the central equatorial pacific a quantitative record of eolian and carbonate deposition detailed records of vso 6i3c percentage and mass accumulation rate of caco and eolian percentage mass accumulation rate and grainsize generated for core rcl l 210 from the equatorial pacific reveal the timing of paleoclimatic events over the past 950 000 yr the caco percentage record shows the standard pacific correlation of high caco content with glacial periods but displays a marked change of character about 490 000 yr ago with older stages showing much less variability the carbonate mass flux record however does not show such a noticeable change sedimentation rates vary from about 0 5 to 3 0 cm l000 yr and during the past 490 000 yr sections with enhanced sedimentation rates correspond to periods of high caco percentage eolian mass accumulation rates an indication of the aridity of the source region are usually higher during glacial times eolian grainsize an indication of the intensity of atmospheric circulation generally fluctuates at a higher frequency than the loo ooo yr glacial cycle the mid brunhes climatic event centered at 300 000 yr ago appears as a 50 000 yr interval of low intensity and reduced variability of atmospheric circulation furthermore the nature of this entire record changes then with the younger portion indicating less variation in wind intensity than the older part of the record the late matuyama increase in amplitude of paleoclimatic signals begins 875 000 yr ago in the eolian record 25 000 yr before the 6i80 and caco percentage amplitude increases about

u th pb dating of collision in the external alpine domains urseren zone switzerland using low temperature allanite and monazite article i nfo dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low temperature metamorphic con ditions in the external domains of the central alps urseren zone the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two ree minerals to date successive metamorphic stages within a collisional cycle the studied outcrop locality of garvera exposes mesozoic metasediments that were metamorphosed under greenschist facies conditions t b 450 c under these conditions ree mineral grains or domains are typically small 5 50 m and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins allanite grains n200 m based on the texture and mineral assemblages ree mineral growth conditions were attributed to be detrital mnz1 syn kinematic mnz2 and aln1 post kinematic at the thermal peak aln2 and retrograde during late stage deformation mnz3 to constrain temporally the successive crystallisation conditions ree minerals have been dated using shrimp ion microprobe u th pb analyses show that the alpine ree minerals have a high th u and while most of the 206 pb is non radiogenic f206 that can reach up to 99 for allanite th pb ages can be obtained using 206 pbc corrected th pb isochron the age of different generations of allanite and monazite constrains the timing of successive collisional stages for the prograde accretion at 22 5 1 5 ma mnz1 and aln1 thermal peak at 19 3 2 0 ma aln2 and late stage hydrothermal veining at13 6 1 4 ma mnz3 these novel geochronological data confirmtheregional diachroneity throughthe cen tral alps with younger ages burial thermal peak and exhumation toward the external domains the similarity betweenmetamorphicagesofsamplestakenintheurserenzoneandthenearbycrystallinemassifssuggeststhat themetasedimentarycoveroftheurserenzoneremainedcloselyjuxtaposedtotheexternalcrystallinebasement during the alpine collision cycle the monazite age of the late stage vein testifies for active tectonic deformation at 13 6 1 5 ma which likely played a crucial control on the exhumation of the external massifs in the middle miocene

transient allanite replacement and prograde to retrograde monazite re crystallization in medium grade metasedimentary rocks from the orlica śnieznik dome czech republic poland textural and geochronological arguments abstract we use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium grade metasedimentary rocks from the european variscan belt orlica nieznik dome czech republic poland the relationships between monazite allanite and other major and accessory phases are characterized for staurolite sillimanite and kyanite grade garnet bearing mica schist samples garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim allanite is virtually absent from the matrix it shows replacement textures involving a cluster of th u y rich silicates or phosphates monazite with frequent micro cracks or trimmed grain boundaries abounds in the mica rich matrix it occurs as lone grains close to partially resorbed garnet apatite or staurolite and as subhedral and compositionally zoned aggregates associated with allanite and xenotime electron microprobe analyses of monazite define pb vs th isochrons ages 363 343 ma with positive intercepts pb 0 40 120 ppm which suggest the presence of initial pb and age overestimation isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial pb which leads to variable discordance in the 206 pb 238 u 207 pb 235 u system conversely the 206 pb 238 u 208 pb 232 th system is not visibly affected analyses are concordant and 208 pb 232 th ages range from 358 to 299 ma 7 8 ma regardless of monazite texture or composition they define a main group at 330 310 ma with few older outliers all observations suggest that a first monazite generation mnzi formed via allanite breakdown at prograde to peak p t conditions 5 7 kbar 575 640 c a comparison with previous geochronological results reveals that the age of this prograde reaction is however preserved by only few domains yielding 360 340 ma instead our results emphasize the importance of retrograde processes which built the dominant age population of 330 310 ma the latter ages are ascribed to a combination of 1 transient monazite growth after allanite and controlled by p availability 2 variable recrystallization replacement of older monazite grains and 3 minor monazite neoformation due to the resorption of garnet or apatite the young monazite grains or domains mnzii mostly formed during fluid influx and chloritization they constrain the end of the orogenic history when mid crustal rocks were exhumed and re hydrated at temperatures below 300 c