silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits an example of meteoric dominated diagenesis from the triassic of central spain cauliflower shaped nodules are widespread in a single red mudstone bed in the buntsandstein triassic facies of the iberian range they consist mostly of quartz dolomite and calcite but other minerals such as barite kaolinite and iron oxyhydroxides are also present the nodules are spherical ovoid or elongate in shape and range from 1 to 8 cm across the surface of the nodules is irregular and some show a pedogenic coating of microspar the sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of ca2 and so4 2 in a vadose environment occasionally under the influence of pedogenic processes partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting as indicated by the presence of organic filaments on the quartz crystal surfaces in type a nodules the dissolution of the innermost anhydrite was complete and different types of quartz cement filled the porosity fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules in type b nodules the dissolution of the anhydrite was incomplete inhibiting quartz cementation and enabling later dolomitization of the anhydrite dolomitization appears to have been driven by sulphate reduction as indicated by the presence of bacterial bodies within the dolomite crystals dedolomitization and precipitation of barite kaolinite and calcite spar cements occurred later under the influence of meteoric solutions the nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions

sedimentology and diagenesis of upper mississippian to lower permian strata talbot lake area jasper national park alberta in the talbot lake area the upper mississippian carbonate sequence comprises in ascending order the turner valley and mount head formations the mount head formation is informally divided into lower and upper members the rocks of the upper mississippian originated by sedimentation associated with successive shallowing upward sequences on the basis of the fossils lithologies textural characters and sedimentary structures the turner valley formation apparently resulted from sedimentation on open marine shoals and in a lagoon the lower member of the mount head formation from sedimentation in shallow subtidal to lower supratidal environments and the upper member of the mount head formation from deposition in upper intertidal to supratidal environments the paraconformity between the lower and upper members of the mount head formation records a short erosional break a disconformity between the upper mississippian carbonates and the upper permian ishbel group displays paleotopographic relief and pale

armoured sponge spicules from panarea island italy implications for their fossil preservation abstract a variety of calcareous and siliceous skeletal components are associated with exceptional still active deposits of modern iron ooids off panarea one of the volcanic islands of the aeolian arc tyrrhenian sea italy both ooids and skeletal components occur as loose sediments influenced by submarine hydrothermal fluid vents whereas iron ooids are exclusively made of goethite laminae primarily nucleated on volcaniclastic material sponge spicules that represent most of the siliceous skeletal component develop laminated coatings concretions of varying sizes this partial or total coating consists of regularly banded fe rich layers exhibiting the same textural features and mineralogical composition goethite of the ooid cortex developed around inorganic cores spicules did not reveal any obvious attaching structure or interfingering with the surrounding coating and did not undergo bioerosion or any other evidence of biological intervention during their development a hydrothermal origin compatible with the general setting is therefore proposed for these armoured sponge spicules we believe that this unique modern case of iron concretions produced on siliceous spicules can contribute to explain other known fossil cases as well as to understand the taphonomy of this type of biogenic silica that seems rarely to have been preserved in such an extreme habitat

younger dryas style cooling events at glacial terminations i vi at odp site 658 associated benthic δ13c anomalies constrain meltwater hypothesis high resolution planktonic and epibenthic stable isotope records from ocean drilling program site 658 off northwest africa provide a basis for a detailed study of glacial terminations i vi during the last 650 000 years the duration of the terminations was about one half to one quarter of an orbital precession cycle 5800 10 700 years when its amplitude was high at low amplitudes the terminations lasted longer than half an obliquity cycle 29 000 years marked climatic rebounds similar to the younger dryas each with a duration of 1000 2500 years subdivided all six terminations into distinct steps a b and c important parts of the deglacial steps were as brief as 700 1000 years the speed of climatic change suggests that special associations existed between orbital forcing and inherent instability of the ice sheets in harmony with published models the more rapid pulses of glacial meltwater incursions to the northern north atlantic led to one or more brief short term shut downs of north atlantic deep water formation this process is reflected by pronounced benthic 13c minima that precisely coincide in most cases with the end of the 18o deglaciation steps and immediately terminate with succeeding younger dryas style cooling events thus we conclude that the rebounds resulted from a short term antecedent estuarine circulation regime in the north atlantic except for the younger dryas itself which succeeded the allerod with its well ventilated atlantic deepwater circulation and hence continues to be an enigma

comment on neutron activation analysis of stone from thechadron formation and a clovis site on the great plains by hoard et al 1992 abstract comments are made on the previously published paper by hoard et al neutron activation analysis of stone from thechadron formation and a clovis site on the great plains journal of archaeological science 19 655 665 1992 a number of questions and their implications are discussed

liquid immiscibility and the evolution of basaltic magma this experimental study examines relationships between alternative evolution paths of basaltic liquids the so called bowen and fenner trends and silicate liquid immiscibility synthetic analogues of natural immiscible systems exhibited in volcanic glasses and melt inclusions were used as starting mixtures conventional quench experiments in 1atm gas mixing furnaces proved unable to reproduce unmixing of ferrobasaltic melts yielding instead either turbid opalescent glasses or crystallization of tridymite and pyroxenes in contrast experiments involving in situ high temperature centrifugation at 1000g g 1 4 9 8 m s did yield macroscopic unmixing and phase separation centrifugation for 3 4 h was insufficient to complete phase segregation and resulted in sub micron immiscible emulsions in quenched glasses for a model liquid composition of the middle zone of the skaergaard intrusion at super liquidus temperatures of 1110 11208c centrifugation produced a thin silicic layer 64 5 wt sio2 and 7 4 wt feo at the top of the main fe rich glass 46 wt sio2 and 21 wt feo the divergent compositions at the top and bottom were shown in a series of static runs to crystallize very similar crystal assemblages of plagioclase pyroxene olivine and fe ti oxides we infer from these results that unmixing of complex aluminosilicate liquids may be seriously kinetically hampered presumably by a nucleation barrier and thus conventional static experiments may not correctly reproduce it in the light of our centrifuge experiments immiscibility in the skaergaard intrusion could have started already at the transition from the lower to the middle zone thus magma unmixing might be an important factor in the development of the fe enrichment trend documented in the cumulates of the skaergaard layered series

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

evidence for silicate liquid immiscibility in monzonites and petrogenesis of associated fe ti p rich rocks example from the raftsund intrusion lofoten northern norway

last interglacial and holocene climatic development in the norwegian sea region ocean front movements and ice core data planktonic foraminiferal evidence suggests that the ocean front systems between polar and atlantic surface waters in the norwegian sea generally were located closer to greenland during oxygen isotope substage 5e than in the holocene during both these periods oscillations have occurred in the position of the fronts in the western norwegian sea region the substage 5e influence of warm atlantic waters was interrupted by a return to polar conditions these findings support both ice core data and evidence from europe that the last interglacial was a period of rapid climatic shifts

dynamics of atlantic water advection to the norwegian greenland sea a time slice record of carbonate distribution in the last 300 ky abstract short term variability in volume and configuration of atlantic water advection to the norwegian greenland sea ngs has recently been identified as a new and significant element for northern hemisphere glacial interglacial climatic fluctuations the amount of salt heat and moisture transferred by atlantic waters to the high arctic effectively influences northern hemisphere climates and is critical for the overall oceanic turnover rate in particular determining the extent and duration of atlantic water intrusions in the ngs is essential because of 1 its effect on the climate of the nordic continents and 2 its trigger function on the stability of the ice sheets with a weak inflows of atlantic water during glacials promoting an increase in ice volume by moisture supply to polar areas and b positive feed back mechanisms of atlantic water intrusions during ice sheet decay in this study the dynamics of surface water mass regimes in the ngs have been reconstructed for the past 300 ky by means of time slice compilations of calcium carbonate distribution and micropaleontologic proxies in addition carbonate preservation records are used to quantify deep water ventilation and production rates in contrast to the climap scenario but in accordance with many recent publications results of this study indicate that atlantic water advection was also a common feature during most glacial periods but varied strongly in volume and configuration significant modes recognized are 1 the full interglacial mode with a wide inflow of warm atlantic waters covering a broad area in the eastern sector of the ngs and extending far north combined with excellent carbonate preservation and very efficient deep water formation stages 1 and 5 5 1 2 the moderate interglacial mode with a rather small extension of atlantic waters along a narrow corridor in the southeastern sector of the ngs but still very efficient deep water exchange stages 9 isotopic events 7 5 7 3 7 1 5 3 and 5 1 3 pronounced cool phases during interglacials with only a very narrow inflow of atlantic waters into the southeastern ngs while the coastal areas off norway were still affected by iceberg drift substages 7 4 and 5 2 carbonate preservation records indicate ongoing deep water formation in the ngs 4 the normal glacial mode characterized by a variable but persistent inflow of cool temperate atlantic water lower stage 8 repetitively during stages 6 4 3 1 and 2 carbonate preservation records indicate ongoing deep water formation in the ngs 5 short glacial and deglacial periods characterized by episodic high inputs of ird evidenced by a wide coverage of diamicton deposition and strong carbonate dissolution e g episodically within stages 10 6 and 2 the 6 5 4 3 and 2 1 transitions some of these ngs diamicton layers correlate with heinrich layers in the north atlantic but generally ngs diamicton layers occur more frequently than heinrich layers the role of atlantic water influence during these events remains enigmatic but atlantic water influence is frequently found just above and below these sections a scenario with rapid ice sheet disintegration induced by atlantic water advection via increase of the thermal regime along the ice margin possibly triggered by small scale sea level rise might be envisaged more obviously the short term cessation of deep water formation in the ngs as evidenced by the dissolution spikes may be correlated with contemporaneous ventilation minima in the north atlantic

development of glacial and interglacial conditions in the nordic seas between 1 5 and 0 35 ma sedimentological and geochemical proxy records of a deep sea sediment core from the southern central nordic seas were used to reconstruct the development of glacial and interglacial conditions during the early and middle pleistocene i e late matuyama to middle brunhes chron 1 5 0 35 ma an enhancement of both glacial and interglacial characteristics is observed during early brunhes oxygen isotope stages ois 16 and 15 respectively any intensification of the climatic conditions prior to this as was previously described for the eastern part of the nordic seas is not recognized at our study site it is further shown that the glacial interglacial environmental contrasts increased from the early to the middle bruhnes chron of all glacial periods investigated ois 12 is characterized by the most severe conditions showing both maximum input of iceberg rafted debris ird as well as planktic foraminiferal 18o values comparable to those of the last glacial maximum among the interglaciations ois 11 is by far the longest interval and the first to show fully developed interglacial conditions i e holocene like 18o values and a minimum of ird deposition hence our comparison supports bottom water 18o studies that have indicated the existence of a gradual intensification of glacial interglacial climate contrasts during the middle pleistocene

evolution of porewater composition through time in limestone aquifers salinity and d h of fluid inclusion water in authigenic minerals jurassic of the eastern paris basin france past water circulations can significantly reduce the porosity and permeability of marine limestones this is particularly the case in the middle bathonian bajocian to upper oxfordian jurassic limestones from the eastern border of the paris basin the knowledge of the timing the temperature and composition of paleowaters is essential to model the hydrological evolution in this area where the callovian oxfordian claystones are studied for the storage of nuclear wastes in this way fluid inclusions hosted in low temperature 60 c authigenic calcite quartz and celestite crystals were analyzed by raman spectroscopy and mass spectrometry to determine the chlorinity and d h ratios chlorinity measurements mmol cl per liter of water in fluid inclusions trapped in authigenic crystals during the late jurassic early cretaceous period revealed unexpected high values up to 3800 mmol l 1 indicating that brines were involved in some of the diagenetic crystallization processes by contrast fluid inclusions in calcite cements of cenozoic age within the oxfordian limestones have low cl concentration less than 150 mmol l 1 thus showing that a dilution event caused by water infiltrations during the cretaceous uplift of this part of the basin has flushed out the original saline porewater by coupling d of fluid inclusion with 18o of calcite crystals we estimate that calcite precipitation occurred at temperatures between 25 and 53 c the hydrogen isotope composition of calcite forming water is different between the middle jurassic d ranging from 20 to 35 8 v smow and the overlying oxfordian limestone d from 59 5 to 44 8 v smow present day groundwaters are also of distinct composition on both sides of the oxfordian claystones indicating that limestone aquifers underwent independent hydrologic evolutions since the early diagenetic jurassic cementation

chert appearance in the cueva bedon carbonate platform upper cretaceous northern spain nodular chert from the cueva bedon carbonate platform late turonian earliest coniacian occurs above the type 1 sequence boundary and in association with a transgressive systems tract deposit it formed by selective replacement of grainstones packstones and thalassinoides burrow fillings other minor types of selective silicification include the replacement of anhydrite nodules forming quartz geodes and the partial or total replacement of oysters and inoceramids the chert nodules consist predominantly of microquartz with volumetrically minor fibrous quartz length fast chalcedony and megaquartz the silica has a mainly biogenic source siliceous sponge spicules and radiolarians lepisphere textures that are indicative of an intermediate opal ct precursor are evident in chert nodules which indicates the chert formed by a maturation process from opala to opal ct to quartz the 18 o values of microquartz suggest a precipitation from marine fluids the fabric of quartz geodes consists of irregular bands of heterometric megaquartz quartzine and lutecite spherulites and a well developed band of petaloidal megaquartz that contains small anhydrite inclusions bivalve shells are replaced mainly by fibrousquartz quartzine lutecite which forms extensive beekite rings the silica source of the replaced fossils is also biogenic but silicification couldhave occurred by direct quartz replacement without an opal ct precursor field and petrographic evidence indicate that silicification was an early diagenetic process that affected both uncompacted and compacted sediments andwas inhibited when the dolomitization began there seems to exist a syncrony in age late turonian earliest coniacian between the nodular chert in the cueva bedon carbonate platform and the bedded and nodular chert ofthe plentzia carbonate turbidites in the deeper basque arc

oceanic evidence for coherent fluctuations in fennoscandian and laurentide ice sheets on millennium timescales proxy temperature records from greenland ice cores1 2 and north atlantic sediment cores3 have provided evidence for a high degree of climate instability during the last glacial period much of this variability seems to be linked with the dynamics of the laurentide ice sheet that covered north america at this time3 which dis charged iceberg flotillas into the north atlantic that are now recorded in sediment cores as heinrich events4 how if at all this variability was manifested on the other side of the atlantic in the nordic seas and the ice sheets of northwest europe and scandinavia has been unclear here we present sediment micro fossil and oxygen isotope data from a sediment core in the norweg ian sea which reveal cooling events and iceberg discharges analogous to heinrich events we show that these climate fluctua tions in the norwegian sea were in phase or were phase locked with air temperatures over greenland suggesting that the rapid changes in heat fluxes in the north atlantic recorded in previous records3 were felt in this high latitude region the iceberg dis charges in our record seem to have come from the fennoscandian ice sheet implying that this and the laurentide ice sheets fluctu ated coherently on timescales shorter than those of milankovitch orbital cycles

pliocene pleistocene ice rafting history and cyclicity in the nordic seas during the last 3 5 myr a continuous 3 5 myr ird record was produced from ocean drilling program odp site 907 a timescale based on magnetic polarity chrons oxygen isotope stratigraphy for the last 1myr and orbital tuning was developed the record documents a stepwise inception of large scale glacial cycles in the nordic seas region the first being a marked expansion of the greenland ice sheet at 3 3 ma a second step occurred at 2 74 ma by an expansion of large scale ice sheets in the northern hemisphere ice sheet variability around the nordic seas was tightly coupled to global ice volume over the past 3 3 myr between 3 and 1 ma most of the variance of the ird signal is in the 41 kyr band whereas the last 1 myr is characterized by stronger 100 kyr variance the gamma ray porosity evaluator grape density record is closely linked with ird variations and documents sub orbital variability resembling the late quaternary heinrich bond cycles

high resolution climate records from the north atlantic during the last interglacial the two deep ice cores recovered by the grip1 and gisp22 projects at summit greenland agree in detail over the past 100 000 years3 and demonstrate dramatic climate variability in the north atlantic region during the last glacial before the current period of holocene stability this glacial climate instability has subsequently been documented in the marine sedimentary record of surface ocean conditions in the north atlantic4 before 100 kyr ago the two ice core records are discrepant however casting doubt on whether the oxygen isotope fluctuations during the last interglacial eemian seen in the grip core1 5 represent a true climate signal here we present high resolution records of foraminiferal assemblages and ice rafted detritus from two north atlantic cores for the interval 65 kyr to 135 kyr ago extending the surface ocean record back to the eemian the correlation between our records and the greenland ice core records is good throughout the period in which the two ice cores agree suggesting a regionally coherent climate response during the eemian our marine records show a more stable climate than that implied by the grip ice core suggesting that localized phenomena may be responsible for the variability in the latter record during the eemian

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

astronomic timescale for the pliocene atlantic δ18o and dust flux records of ocean drilling program site 659 high resolution benthic oxygen isotope and dust flux records from ocean drilling program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the atlantic from 2 85 ma back to 5 ma spectral analysis of the 18o record indicates that the 41 kyr period of earth s orbital obliquity dominates the pliocene record this is shown to be true regardless of fundamental changes in the earth s climate during the pliocene however the cycles of sahelian aridity fluctuations indicate a shift in spectral character near 3 ma from the early pliocene to 3 ma the periodicities were dominantly precessional 19 and 23 kyr and remained strong until 1 5 ma subsequent to 3 ma the variance at the obliquity period 41 kyr increased the timescale tuned to precession suggests that the pliocene was longer than previously estimated by more than 0 5 m y

silicate liquid immiscibility within the crystal mush late stage magmatic microstructures in the skaergaard intrusion east greenland late stage microstructures developed during the last stages of solidification of the skaergaard intrusion comprise a wide array of reactive and non reactive intergrowths reactive microstructures ascribed to open system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts fish hook pyroxenes and mafic symplectites they form by the addition of fe and ca and removal of alkalis and silica other microstructures include those formed by internally generated redox reactions between olivine fe ti oxides and pyroxene non reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids the open system reactive microstructures occur predominantly in the cumulates on the chamber floor appearing in lzb and disappearing to be replaced by a granophyre ilmenite rich intergrowth association in the upper zone they are not common in the marginal border series their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a si rich immiscible component from the interstitial liquid the loss of the si rich component causes the remaining fe rich liquid to react with the primocrysts bulk rock major element data are consistent with little or no preferential loss of the si rich liquid from the mush but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma the first appearance of the paired conjugate non reactive intergrowths in mz marginal border series points to the early onset of immiscibility in the bulk liquid

prediction of plagioclase melt equilibria in anhydrous silicate melts at 1 atm many models for plagioclase melt equilibria have been proposed over the past 30 years but the focus is increasingly on the effects of water content and pressure however many geological and petrological applications concern low pressure and low water systems such as the differentiation of large terrestrial basaltic magma chambers and lunar and asteroidal magmatism there is therefore a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1 atm with this in mind a database of over 500 experimentally determined plagioclase liquid pairs has been created the selected low pressure anhydrous experiments include both natural and synthetic liquids whose compositions range from basalt to rhyolite four equations are proposed derived from this data the first is based on a thermodynamically inspired formalism explicitly integrating the effect of temperature this equation uses free energies and activities of crystalline anorthite available from the literature for the activity of anorthite in the liquid phase it is found that current models of the activity of individual oxides are insufficient to account for the experimental results we have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition based upon inversion of the experimental data using this expression allows the calculation of plagioclase composition with a relative error less than 10 however in light of the fact that temperature is not necessarily known for many petrological applications an alternative set of t independent equations is also proposed for this entirely empirical approach the database has been divided into three compositional groups treated independently for regression purposes mafic ultramafic alkali rich mafic ultramafic and intermediate felsic this separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits but results across group boundaries were found to be comparable overall 50 of plagioclase compositions are predicted to within 2 of the experimentally derived value and 90 to within 5 representing a significant improvement over existing models

seven million years of glaciation in greenland glacial till glaciomarine diamictites and ice rafted detritus found in marine cores collected off the shore of southeast greenland record multiple late cenozoic glaciations beginning in the late miocene distinct rock assemblages and seismic stratigraphic control correlate the diamictites with glaciation of the southeast greenland margin glaciers advanced to the sea during several intervals in the pliocene and pleistocene north atlantic glaciation may have nucleated in southern greenland rather than further north because of the high mountains and the high levels of precipitation in this region

depositional regimes in the norwegian greenland sea the last two glacial to interglacial transitions various models of surface and deep water circulation in the norwegian greenland sea ngs have been proposed for the last two glacial to interglacial transitions although much progress has been made in understanding the sedimentary response to climatic and oceanographic changes conflicting interpretations have been developed to clarify some of these discrepancies and to test or modify the existing circulation concepts a multiparameter approach is applied combining sedimentological micropaleontological organic geochemical and isotopic methods on the basis of indicative properties a combined litho and organofacies concept is developed and calibrated with modern depositional settings beneath different surface water masses sedimentary regimes are then derived for glacial and deglacial settings

precipitation of silica and formation of chert mudstone peat association in miocene coastal environments at the opening of the sea of japan abstract a sedimentary sequence of layered chert alternating with mudstone and associated peat is present at yachi japan these deposits were accumulated in a local lacustrine to brackish water environment along a coastal setting during the early to middle miocene the basement of the sedimentary sequence is rhyolite erupted just before the chert precipitation and its weathered derivatives the chert and the mudstone lack coarse detrital grains on the other hand associated peat contains abundant plant leaves and debris that are compressed and deformed post depositionally uncompressed leaf fragments and broken twiggy trunks in the chert suggest rapid chert precipitation and early lithification four quartz fabrics are recognized chalcedony microquartz megaquartz and euhedral quartz these types vary from colorless to pale brown microscopic sem and xrd observations reveal a paragenetic sequence of silica crystallization chalcedony microquartz and megaquartz was precipitated and consolidated rapidly then euhedral quartz was crystallized in voids there is neither evidence indicating opal a and opal ct precursors of these quartz nor evidence showing that the chert is a post depositional silicification product of volcanic ashes petrographic observation indicates that the initial silica mineral phase was quartz and chalcedony silica was interpreted to have precipitated as a result of influx of low ph water and mixing with silica concentrated solution in the coastal depressions the petrography and depositional environment of the chert are compared with those of other chert types the chert mudstone peat association found at the yachi seems to represent brackish water sediments deposited during continental rifting of the sea of japan type

late cenozoic seismic stratigraphy and glacial geological development of the east greenland and svalbard barents sea continental margins abstract the european program on the late cenozoic evolution of the polar north atlantic margins ponam running from 1990 to 1994 resulted in a wealth of seismic data and sediment cores particularly from the svalbard and barents sea continental margin but also from areas of the east greenland margin deep sediment coring was undertaken on both margins during odp legs 152 and 162 this paper uses the ponam generated information as well as previous and subsequent seismic and core data to review our present understanding of the glacial geological development of these regions four main depositional facies have been identified these are represented by hemipelagic glaciomarine sediments sandy silty debris flows diamictic debris flows and tills deposited directly beneath grounded ice the temporal and spatial distribution of these four facies determines the architecture and seismic character of the two continental margins the two regions show significant differences with regard to both the timing of initial glaciation and the response of the ice sheets to climatic variability glacial deposition has taken place on the east greenland margin at least since 7 ma but apparently only since 2 5 ma on the svalbard barents sea margin significant glacial expansion across the continental shelf probably occurred in the early and late pliocene in east greenland whereas the glaciers on svalbard and the barents sea did not reach the shelf edge until the early pleistocene despite a shorter glacial period average glacial sedimentation rates are 2 3 times higher on the svalbard barents sea margin than on the studied parts of the east greenland margin the svalbard barents sea ice sheet has advanced and retreated much more frequently during the plio pleistocene than the greenland ice sheet glacial regime polar on greenland and polythermal to temperate in svalbard and the barents sea and the more easily erodible preglacial bedrock of svalbard and the barents sea are thought to be main factors responsible for the differences in ice sheet behavior and consequently glacial sediment production

reflection of scandinavian ice sheet fluctuations in norwegian sea sediments during the past 150 000 years the record of glacier fluctuations in western scandinavia as reconstructed from continental data has been correlated with records of ice rafted detritus ird from well dated sediment cores from the norwegian sea covering the past 150 000 yr b p the input of ird into the ocean is used as a proxy for ice sheet advances onto the shelf and thus for the calibration of a glaciation curve the marine results generally support land based reconstructions of glacier fluctuations and improve the time control on glacial advances the saalian ice sheet decayed very rapidly approximately 125 000 yr b p in the early weichselian a minor but significant ird maximum indicates the presence of icebergs in isotope substage 5b especially between 95 000 and 83 000 yr b p reduced amounts of calcareous nannofossils indicate that surface waters were influenced by meltwater discharges during isotope substages 5d and 5b an extensive build up of inland ice began again during isotope stage 4 but maximum glaciation was reached only in early stage 3 58 000 53 000 yr b p marine sediments have minimum carbonate content indicating strong dilution by lithogenic ice rafted material generally the ird accumulation rate was considerably higher in stages 4 2 than in stage 5 a marked peak in ird accumulation rates from 47 000 to 43 000 yr b p correlates well with a second middle weichselian ice sheet advance dated by the laschamp olby paleomagnetic event minimum ice extent during the alesund interstade 38 500 32 500 yr b p and several glacial oscillations during the late weichselian are also seen in the ird record of several late weichselian glacial oscillations on the shelf at least four correspond to the north atlantic heinrich events ice sheet behavior was either coupled or linked by external forcing during these events whereas internal ice sheet mechanisms may account for the noncoherent fluctuations

extreme iron enrichment and liquid immiscibility in mafic intrusions experimental evidence revisited abstract this paper examines phase equilibria and mass balance constraints on fe enrichment in tholeiitic liquids in plutonic environments the peak of fe enrichment is thought to roughly coincide with magma saturation in fe ti oxides and olivine starts to react with the liquid at about the same time this crucial stage of crystallization is examined in detail using a compilation of chemical analyses of 64 experimental charges that comprise liquids quenched glasses equilibrated with the liquidus assemblage of olivine plagioclase high ca pyroxene and low ca pyroxene some samples also contain fe ti oxides it is shown that the 4 phase liquidus assemblage does not constrain a narrow range of liquid compositions the concentrations of sio 2 in the selection of experimental glasses vary broadly from 42 to 66 wt silica content shows strong negative correlations with feo and cao al 2 o 3 and strong positive correlation with alkalis extreme fe enrichment above 22 wt feo is observed only in alkali free or alkali poor liquids broad compositional variations for the multiply saturated liquids are attributed to strong non ideality and complex concentration activity relationships in ferrobasaltic melts liquid immiscibility characteristic of fe rich silicate liquids is the ultimate consequence of non ideality petrogenetic implications of phase equilibria and mass balance constraints are discussed for a classical example of the skaergaard intrusion in east greenland where the trend of extreme fe enrichment has been in contention it is proposed that seemingly conflicting results of experiments on skaergaard natural cumulate rocks and model melt compositions can be reconciled if it is assumed that silicate liquid immiscibility in skaergaard started not at the very end of crystallization but earlier soon after the start of ilmenite and magnetite crystallization

multistage hydrothermal silicification and fe tl as sb ge ree enrichment in the red dog zn pb ag district northern alaska geochemistry origin and exploration applications geochemical analyses of major trace and rare earth elements ree in more than 200 samples of variably silicified and altered wall rocks massive and banded sulfide silica rock and sulfide rich and unmineralized barite were obtained from the main aqqaluk and anarraaq deposits in the red dog zn pb ag district of northern alaska detailed lithogeochemical profiles for two drill cores at aqqaluk display an antithetic relationship between sio2 al2o3 and tio2 zr which together with textural information suggest preferential silicification of carbonate bearing sediments data for both drill cores also show generally high tl sb as and ge and uniformly positive eu anomalies eu eu 1 0 similar high tl sb as ge and eu eu values are present in the footwall and shallow hanging wall of zn pb ag sulfide intervals at anarraaq but are not as widely dispersed net chemical changes for altered wall rocks in the district on the basis of average al normalized data relative to unaltered black shales of the host kuna formation include large enrichments 50 of fe ba eu v s co zn pb tl as sb and ge at both red dog and anarraaq si at red dog and sr u and se at anarraaq large depletions 50 are evident for ca at both red dog and anarraaq for mg p and y at red dog and for na at anarraaq at both red dog and anarraaq wall rock alteration removed calcite and minor dolomite during hydrothermal decarbonation reactions and introduced si eu and ge during silicification sulfidation reactions deposited fe s co zn pb tl as and sb barite mineralization introduced ba s and sr light ree and u were mobilized locally this alteration and mineralization occurred during mississippian hydrothermal events that predated the middle jurassic cretaceous brookian orogeny early hydrothermal silicification at red dog took place prior to or during massive sulfide mineralization on the basis of the dominantly planar nature of zn pb veins which suggests filling of fractures that developed in previously lithified rock uniformly low ca and mg and uniformly negative ce anomalies in highly siliceous red dog wall rocks reflect hydrothermal decarbonation reactions and pervasive silicification owing to conductive cooling of oxidized metalliferous fluids similar ca and mg depletions are evident at anarraaq but generally lack associated silicification possibly because temperatures of the hydrothermal fluids were too low 180 c or because the thermal contrast between the fluids and wall rocks was smaller owing to the greater depth of alteration and mineralization there compared with red dog chalcophile element anomalies fe zn pb tl as sb in wall rocks at both red dog and anarraq are attributed to sulfidation reactions coeval with subsurface zn pb ag mineralization during the mixing of oxidized metalliferous fluids with h2s rich fluids derived locally within the kuna formation sedimentary wall rocks in the red dog district are characterized by a distinctive suite of geochemical anomalies especially for zn pb tl as sb ge and eu eu at the aqqaluk deposit wall rocks without visible sphalerite or galena 300 ppm zn pb have anomalous eu eu tl sb and as for up to 100 m stratigraphically below zn rich silica rock at anarraaq the tl anomaly is most extensively developed and enrichment relative to unaltered black shale of the kuna formation is present up to 62 m above the highest zn pb sulfide zones the magnitude of the enrichment and systematic behavior of tl in the district make tl a promising geochemical exploration guide for red dog type zn pb ag deposits elsewhere

surface and deep water changes in the subpolar north atlantic during termination ii and the last interglaciation to reconstruct the history of water mass exchange between the ne atlantic and the nordic seas sediment cores from 2 km water depth were studied across termination ii tii and through the last interglaciation mis5e during early tii the sudden appearance of the low latitude planktonic foraminifera beella megastoma is noted in both regions along with a steep decrease in benthic foraminiferal 18o since other proxies indicate that surface waters were cold and stratified because of meltwater conditions which prevented near surface thermohaline circulation and vertical convection in the nordic seas water mass exchange between the two areas occurred at the subsurface during later tii surface conditions changed and this subsurface circulation style was eventually replaced by vertical convection in the nordic seas b megastoma vanished from the record together with ice rafted debris ird at the end of tii while subpolar foraminiferal abundance rose peak interglacial conditions with intensive vertical convection now fully developed generating a bottom water temperature gradient of 4 c between the two areas however surface water temperatures deteriorated in the nordic seas already notably before ird recurred and 18o increased at the end of mis5e

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

dual origin of fe ti p gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the sept iles layered intrusion we present a detailed study of two ca 200 m thick apatite bearing ferrogabbro horizons of the sept iles layered intrusion canada these rocks are the most evolved cumulates of the megacyclic units mcu i and ii and mark the transition between basaltic and silicic magmatism they are made up of plagioclase an55 34 olivine fo66 21 clinopyroxene mg 75 55 ilmenite magnetite apatite pigeonite and are a significant source of fe ti p ore ferrogabbros have relatively uniform bulk rock compositions in mcu i but are bimodal in mcu ii the liquid lines of descent for major elements in equilibrium with cumulates of mcu i and ii have been calculated using a forward model formalism both trends evolve towards sio2 enrichment and feot depletion after saturation in fe ti oxides however because of magma mixing in mcu ii they do not follow the same path evolved liquids from mcu ii are shown to enter the experimentally determined two liquid stability field while mcu i liquids do not immiscibility in mcu ii and its absence in mcu i are supported by the presence of contrasted reactive symplectites in cumulate rocks apatite bearing ferrogabbros in mcu ii have crystallized from distinct immiscible fe rich and si rich silicate melts which have physically segregated in the slow cooling magma chamber two different types of cumulate rocks are thus produced leucocratic and melanocratic gabbros this is consistent with the presence of si rich and fe rich melt inclusions in apatite in contrast homogeneous ferrogabbros from mcu i were produced by simple fractional crystallization of a homogeneous liquid our data suggest that immiscibility could also explain the large geochemical variability of ferrogabbros in the upper zone of the bushveld complex south africa

evidence for a mid pleistocene change of ice drift pattern in the nordic seas sediment proxy records from a continuous 1 5 million year long deep sea sediment core from a site in the western norwegian sea were used to obtain new insights into the nature of palaeoceanographic change in the northern north atlantic nordic seas during the climatic shift of the mid pleistocene revolution mpr red green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr 700 ka with those from the past 500 kyr the timing and duration of these changes indicates that the mpr in the nordic seas is characterised by a gradual transition lasting about 200 kyr together with further sedimentological evidence this suggests that the mid pleistocene climate shift was accompanied by a general change in ice drift pattern it is further proposed that prior to the onset of the major late pleistocene glaciations in the northern hemisphere a significant proportion of the ice in the eastern nordic seas originated from a southern provenance whereas later it dominantly came from the surrounding landmasses

evidence for a steeper eemian than holocene sea surface temperature gradient between arctic and sub arctic regions sediment proxy data from the norwegian greenland and iceland seas nordic seas are presented to evaluate surface water temperature sst differences between holocene and eemian times and to deduce from these data the particular mode of surface water circulation records from planktic foraminiferal assemblages caco3 content oxygen isotopes of foraminifera and iceberg rafted debris form the main basis of interpretation all results indicate for the eemian comparatively cooler northern nordic seas than for the holocene due to a reduction in the northwardly flow of atlantic surface water towards fram strait and the arctic ocean therefore the cold polar water flow from the arctic ocean was less influencial in the southwestern nordic seas during this time as can be further deduced from the eemian data slightly higher eemian ssts are interpreted for the western iceland sea compared to the norwegian sea ca south of 70 n this eemian situation is in contrast to the holocene when the main mass of warmest atlantic surface water flows along the norwegian continental margin northwards and into the arctic ocean thus a moderate northwardly decrease in sst is observed in the eastern nordic seas for this time causing a meridional transfer in ocean heat due to this distribution in ssts the holocene is dominated by a meridional circulation pattern the interpretation of the eemian data imply a dominantly zonal surface water circulation with a steep meridional gradient in ssts

silicon isotope compositions of euhedral authigenic quartz crystals implications for abiotic fractionation at surface temperatures abstract silicon si isotopes have been demonstrated as proxies for the paleoenvironmental conditions of various silica deposits in an effort to investigate the petrogenesis and paleoenvironments of phanerozoic silica deposits a suite of euhedral megaquartz crystals in the cretaceous edwards formation from central texas was petrographically analyzed and their si isotopic compositions analyzed by bulk and in situ techniques petrographic analysis shows a close relationship between megaquartz and evaporite minerals the lithologic association of silica with evaporite bearing dolomitized carbonate strata suggests that the silicification probably developed in a back reef tidal flat environment in which quartz crystals formed after the primary calcite cementation and partial dissolution of evaporites silicon isotopic mapping across the megaquartz crystals show that 30si nbs28 values range from 2 90 to 2 94 these values span the majority of published silicon isotopic values observed in nature and indicate complex growth histories the negative 30si values are attributed to the dissolution of sponge spicules which likely act as the primary source of the authigenic megaquartz the observed range of 30si values in megaquartz crystals is interpreted using a two stage model in which amorphous silica from sponge spicules is dissolved and re precipitated as megaquartz in a closed system during diagenesis this rayleigh type fractionation model also adds a new level of insight into the abiotic fractionation factor between dissolved and precipitated silica based on temperature estimates of 20 to 50 c for megaquartz precipitation the fractionation factor was determined to be between 1 8 and 2 1 the estimated average 30si value of early cretaceous seawater is 2 7 to 3 0 significantly higher than modern seawater

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

low pressure phase relationships in the system na2o cao feo mgo al2o3 sio2 at 1100 c with implications for the differentiation of basaltic magmas

ordovician aleman ribbon cherts an example of silicification prior to carbonate lithification abstract the ordovician aleman formation southwestern new mexico is primarily composed of dolomite with ribbons of chert the presence of stromatolites relict evaporite nodules and other features indicates an intertidal to shallow subtidal depositional environment the stromatolitic layers are preferentially silicified with dissolution of opaline sponge spicules providing the main if not sole source of silica silicification occurred in both the vadose and phreatic realms essentially penecontemporaneous with accumulation of the sediment and concomitant with dolomitization this is indicated by the preservation in silicified form of original anhydrite lath morphology central cavities in chert nodules partially filled with geopetal matrix dolomite and matrix dolomite immediately adjacent to conchoidal fractures of silicified carbonate grainstones the latter relationship indicates that the chert was rigid and fractured before the carbonate host sediments were lithified

a survey of paleozoic microbial fossils in chert abstract non acritarch microbial fossils from paleozoic marine cherts exhibit generally poor preservation with rare exceptional preservation microbial microfossils were discovered in 11 out of 17 stromatolitic and non stromatolitic paleozoic units containing replacement cherts from the continental united states many microfossils likely have a cyanobacterial affinity having both coccoid and filamentous morphotypes examined units range in age from middle cambrian to late permian and were deposited in peritidal open shelf and basinal settings all of which have preserved microbial assemblages cherts from the examined units generally formed during early diagenesis and the quality of microbial preservation does not seem to have changed during the paleozoic which is a period that experienced significant secular changes in the mode of marine silica formation

implications of stratigraphic and paleoclimatic records of the last interglaciation from the nordic seas climatic reconstruction of glacial to interglacial episodes from oxygen isotopes in sediment cores from the nordic seas is complicated by strong local meltwater contributions to the oxygen isotope changes combination of benthic and planktic foraminiferal isotope data with foraminiferal abundances and ice rafted debris ird allows separation of local and global effects and subdivision of the marine oxygen isotope events 6 2 5 4 which include the last interglaciation into 1 a meltwater phase after glacial stage 6 recorded by large amounts of ird and low foraminiferal abundance indicating surface water warming 2 an ird free period with high deposition rates of subpolar foraminifera and other caco3pelagic components recognized here as the full interglaciation and 3 a phase with the recurrence of ird and the demise of subpolar species comparison of ice core records and marine data implies that the global climate during the last full interglaciation and that during the postdeglacial holocene were similar the records show no significantly different variations in the proxy data in contrast the oxygen isotopes of planktic foraminifera and ice cores indicate significant differences during each of the deglacial transitions terminations i and ii that preceded these two interglaciations these suggest that during termination ii the climatic evolution in the nordic seas was less affected by abrupt changes in ocean atmosphere circulation than during the last glacial to interglacial transition

eemian and early weichselian 140 60 ka paleoceanography and paleoclimate in the nordic seas with comparisons to holocene conditions paleoceanographic and paleoclimatic time series were extracted from norwegian iceland and greenland sea the nordic seas sediment sequences covering the time intervals 60 140 ka and 0 15 ka on the basis of stable isotope stratigraphy planktonic foraminiferal faunas and sedimentological methods the proxy temperature records from the norwegian iceland sea display a number of high frequency and high amplitude variations within the last interglacial the eemian indicating marked changes in heat flux at high northern latitudes eemian holocene comparisons document that the climate of the norwegian iceland sea in the last interglacial was less stable than that of the present interglacial and that the distribution of surface water masses in the eemian differed from that of the holocene within the greenland sea uniform cold conditions prevailed throughout both interglacials cooling of the nordic seas as well as the initial growth of the surrounding ice sheets began within the eemian interval before the main increase in global ice volume heat flux to the nordic seas was significantly higher during the interglacials than at any time during the early weichselian 60 113 ka and no marked stadial interstadial changes in sea surface temperature and deep water ventilation are documented in the early weichselian records contrary to the pattern found in north atlantic sediment records however fluctuations in planktonic foraminiferal content planktonic 13c and ice rafting on suborbital timescales mostly comparable in number and age to the stadial interstadial fluctuations in the greenland ice cores are demonstrated in our records the ice rafted detritus records provide a more detailed account of ice sheet variability than previously known for the early weichselian period and demonstrate that glaciers advanced to sea level somewhere around the nordic seas several times during this interval

nodular chert formation in carbonate rocks mapping of carbonate dissolution in nodular chert bearing strata indicates that nodular chert formation in a diverse suite of phanerozoic limestones occurred in bulk pore waters that were at calcite saturation opal ct and quartz supersaturation was achieved in most chert bearing strata by the intraformational dissolution of amorphous silica skeletal material chertification occurred by a combination of force of crystallization controlled replacement of the host carbonate whereby non hydrostatic stresses resulting from opal ct and quartz crystal growth caused calcite dissolution and the isomineralic heterogenous nucleation of new opal ct and quartz crystal at nodule peripheries the restriction of the solution phase at silica carbonate contacts to thin films permitted the preservation of ghosts of micron sized features in the chert heterogeneities in sediment organic matter content porosity and biogenic silica concentrations promote the nucleation of chert nodules in some formations

paragenesis of silicified mid paleozoic and mid cenozoic corals based on petrography and silicon isotopic analyses abstract petrographic analyses of silicified coral fossils from the upper ordovician cutter formation and silurian fusselman formation of new mexico and the paleogene lower neogene tampa member and suwannee limestone of florida show that septal voids of these coral fossils are filled with megaquartz and septal walls are replaced by chalcedonic microspherulites and microquartz the boundaries of the microquartz and spherulites of the septal walls conform to accommodation space left by the megaquartz that filled the septal voids these textures suggest that megaquartz filled the septal voids prior to silicification of the septal walls the measured 30si compositions of silica defining the septal walls and silica that fills the septal voids range from 5 23 to 1 03 the void filling silica has isotopically light si and the septal wall replacement si is isotopically heavier the silicon isotopic data are consistent with distillation of a finite silica reservoir which favors incorporation of lighter silicon isotopes in earlier formed silica by rayleigh fractionation distillation of light isotopes during progressive silica precipitation is also documented in vug filling silica thus silica replacement textures and silica isotopic compositions of samples from different geologic ages and locations regardless of crystal sizes or degree of cementation of the primary porosity in the corals support early void filling silica precipitation followed by septal wall silica precipitation in essentially closed system environments

the effect of cao on the partitioning behavior of ree y and sc between olivine and melt implications for basalt carbonate interaction processes abstract the partitioning of ree y and sc r3 between olivine and melt has been investigated experimentally during basalt carbonate interaction three synthetic basalts meltmg 72 meltmg 75 and meltmg 78 were doped with 0 10 and 20 wt caco3 and then equilibrated for 72 h at 1 atm 1 150 1 200 and 1 250 c and the qfm oxygen buffer the thermal decomposition of caco3 produced cao contents in the melt up to 22 wt regular relationships are found between the ionic radius and the partition coefficient dr3 showing typical near parabolic patterns dr3 is weakly dependent on temperature but decreases with increasing caco3 in the starting material e g dsc decreases from 0 20 to 0 13 from the point of view of the lattice strain theory dr3 is described in terms of the radius of the crystal site r0 the young modulus e due to the elastic response of that site to lattice strain caused by cation insertion and the strain free partition coefficient d 0 3 the value of r0 decreases as ca cations are accommodated into the more distorted m2 site of olivine via progressive ca fe substitutions this mechanism is accompanied by a higher proportion of mg cations entering into the smaller m1 site making the optimum ionic radius smaller and favoring the crystallization of more forsteritic olivines from decarbonated melts the enrichment of ca in the crystal lattice is also proportional to the number of si and ca cations available in the melt this causes e to be anticorrelated either with ca in olivine or the activity of cao in the melt r3 cations behave as network modifiers and during basalt carbonate interaction the increasing abundance of non bridging oxygens enhances the solubility of ree y and sc in the melt as a consequence d 0 3 is negatively correlated with the degree of melt depolymerization additionally the strain of the crystal lattice dominates the dr3 parabolic patterns and d 0 3 is strongly controlled by forsterite and aluminium concentrations in olivine the accommodation of ree y and sc in the crystal lattice requires maintenance of local charge balance by the generation of vacancies in accord with a paired substitution of r3 and a vacancy for mg in octahedral sites

accreted mesozoic pelagic seamount in chirai quarry hokkaido japan petrographic evidence of flint limestone cycles abstract the jurassic to cretaceous cyclic sequences of nodular flints and micritic limestones of the nikoro group in chirai quarry eastern hokkaido were studied petrographically and geochemically they were compared with the cretaceous to eocene cycles of nodular flints and pelagic carbonates on the crests of three pelagic seamounts in the north pacific the nodular flints and micritic limestones in chirai quarry were shown to be deposited in a pelagic environment from the fe mg plots of the non calcareous fractions the flint nodules were formed due to the dissolution of pelagic carbonates and three kinds of dissolution are possible 1 dissolution by deep bottom waters 2 dissolution by organisms to form burrows and 3 dissolution by meteoric groundwaters although nodules formed due to 2 and 3 can be recognized at some horizons in the quarry most of the nodules are interpreted to be formed by dissolution due to upwelling bottom waters on the crest of the seamount the nodules on the crests of modern pelagic seamounts in the pacific are restricted in distribution on the seamount to places where the water depth on the crest at the time of deposition was in the range 2 4 km this may suggest that the upwelling bottom water undersaturated with calcium carbonate is not found at depths shallower than 2 km the cycles of flints and pelagic carbonates are interpreted to denote the facies on the crest of the pelagic seamount therefore the jurassic to cretaceous cycles found in chirai quarry can be recognized as an accreted pelagic seamount

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

evolution of the norwegian current and the scandinavian ice sheets during the past 2 6 m y evidence from odp leg 104 biogenic carbonate and terrigenous records abstract records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2 6 m y of odp sites 643 and 644 in order to reconstruct surface and deep water regimes in the norwegian sea surface water regimes record long lasting moderate glacial conditions during the interval 2 6 1 0 ma small intrusions of atlantic water episodically penetrated into the norwegian sea forming a narrow tongue along the eastern margin which is documented at site 644 the polar front was most probably situated between the site 644 and 643 locations on the outer voring plateau during these time intervals deep water regimes reflect long term persistent corrosive bottom waters most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters deep water production in the norwegian greenland sea may have operated in a different way e g brine formation during winter sea ice growth bottom waters were oxygenated throughout the entire period and deep water was exchanged persistently with the north atlantic increased glacial interglacial environmental contrasts are documented reflecting a strengthening of the norwegian current and intensified glaciations on the surrounding land masses during the interval 1 0 0 6 ma during this time a major shift in the mode of deep water production occurred the onset of large amplitudes in glacial interglacial environmental conditions with maximum contrasts in surface water regimes different modes of deep water production and intensified exchange with the north atlantic marks the last 0 6 ma a broad development of the norwegian current is observed during peak interglacials while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions

reconstruction of glaciation over the past 6 myr from ice borne deposits in the norwegian sea it is well known that a significant intensification of northern hemisphere glaciation occurred 2 5 myr ago in contrast to the much earlier onset 35 myr of glaciation in antarctica1 3 much less is known about the behaviour of the climate before 2 5 myr and it has remained unclear when sizeable glaciers first started to develop in the northern hemisphere here we deduce the history of high northern latitude glaciation over the past 6 myr from records of ice borne deposits in deep sea sediments of the norwegian sea we find that glaciers large enough to reach sea level were present in the norwegian sea area as early as 5 5 myr three million years before the intensification of glaciation at 2 5 myr fluctuations in ice volume can be deduced from the oxygen isotope record but this provides only a global average which may not reflect the history of ice sheet growth in specific regions from a comparison of the oxygen isotope records with the record of ice rafted material we derive an estimate of the relative contributions of southern and northern hemisphere glaciation to global variations in ice volume

silicification of sulphate evaporites and their carbonate replacements in eocene marine sediments tunisia two diagenetic trends samples of chert nodules diagenetic carbonates and evaporites gypsum anhydrite collected from the gypsiferous limestones of the kef eddour member ypressian priabonian near metlaoui and sehib tunisia show selective silicification with great variety in the silicified by products based on d 13 c values which support an organic origin for the carbon carbonates replaced evaporites microbially through bacterial sulphate reduction observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites anhydrite gypsum in the absence of early diagenetic carbonate phases silica with d 18 o values from 25 to 28ae6 standard mean ocean water smow replaced the outer parts of anhydrite nodules at ph 9 in the innermost parts of the anhydrite nodules prevented silica precipitation the record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus the microbial diagenetic replacement of evaporites bacterial sulphate reduction by carbonates calcite aragonite and dolomite favoured silica replacement of carbonates rather than evaporites silica with d 18 o signature of 21 to 26 smow replaced carbonates on a volume for volume basis yielding a more siliceous groundmass and accounting for 90 95 of the nodules the relatively higher d 18 o values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed meteoric marine to marine water the lower d 18 o values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage the silica supply for chert formation could be derived from the reworked bio siliceous deposits diatomites to the west of the basin vestiges of an opal ct precursor undetectable by x ray diffraction xrd were revealed by d 29 si magic angle spinning nuclear magnetic resonance investigations diagenesis of the extraformational and overlying clay rich beds the host limestones are clay poor as shown by xrd measurements and minor volcanogenic and hydrothermal contributions during early diagenetic stages

experimental investigation of the stability of clinopyroxene in mid ocean ridge basalts the role of cr and ca al abstract the change in the stability field of clinopyroxene in mid ocean ridge basalt morb as a function of pressure has been used widely as a geobarometer based on results from crystallization experiments using morb like compositions it has been suggested that morb differentiation occurs at relatively high pressures at ultraslow and slow spreading ridges however differentiation requires the loss of substantial heat and it is unclear how this is possible at elevated pressures to better understand the controls on the stability field of clinopyroxene in morb like compositions we report a series of experiments performed at 0 1 mpa in which the temperature of clinopyroxene saturation was determined in melts with variable cr ca al and f o 2 the results show that increased cr and ca al lead to an expansion of the clinopyroxene stability field incorporating these results into a new model of morb differentiation shows that realistic parental melt cr contents can increase the temperature at which clinopyroxene saturation occurs relative to assuming a cr free melt as is commonly the case likewise high ca al melts will saturate clinopyroxene earlier than low ca al melts and their crystallization may provide an explanation for high mg clinopyroxene in oceanic gabbros the newly calibrated geobarometer gives lower crystallization pressures for morb at the slow spreading swir than previous calibrations but still suggests relatively higher pressures of crystallization with decreasing spreading rate

paleoenvironmental changes in the norwegian sea and the northeast atlantic during the last 2 8 m y deep sea drilling project ocean drilling program sites 610 642 643 and 644 sedimentological isotopic and magnetostratigraphic investigations of ocean drilling program and deep sea drilling project sites 642 643 644 and 610 document the oceanographic and climatic evolution of the norwegian sea and the northeastern atlantic over the last 2 8 m y the results show that a major expansion of the scandinavian ice sheet to the coastal areas took place at about 2 56 ma relatively severe glacials appeared until about 2 ma the period 2 6 1 2 ma experienced in general cold surface water conditions with only a weak influx of temperate atlantic water as compared with late quaternary interglacials the norwegian sea was a sink of deep water through this period but deepwater ventilation was reduced and calcite dissolution was high compared with the holocene deep water formed by other mechanisms than it does today between 2 and 1 2 ma the glaciations in scandinavia were small a transition toward larger glacials took place during the period 1 2 to 0 6 ma corresponding to warmer interglacials and reduced calcite dissolution only during the last 0 6 m y has the oceanographic and climatic system of the norwegian sea varied in the manner described in previous studies of the late quaternary a strong thermal gradient was present between the norwegian sea and the northeastern atlantic during the matuyama 2 5 0 7 ma this is interpreted as a sign of a more zonal and less meridional climatic system over the region compared with the present situation

evidence from fram strait 78 n for early deglaciation recent syntheses of the history of the last northern hemisphere glaciation and deglaciation illustrate that understanding of the mechanisms and timing of deglaciation before approximately 12 000 years bp1 5 is limited after 12 000 yr bp however there is sufficient evidence from radiocarbon dated moraines raised beaches varved lake sediments and pollen records to provide a reasonable temporal and geographic picture of the decay of the ice sheets here we report on the first oxygen isotope record from the norwegian greenland sea that is radiocarbon dated directly by accelerator mass spectrometry a significant light oxygen isotope event is recorded at approximately 15 000 years bp which suggests that the marine based barents shelf ice sheet disintegrated rapidly at this time recent studies6 7 have estimated that the decay of this ice sheet could have contributed as much as 15 metres to eustatic sea level rise the decay of the barents shelf ice sheet is the earliest major deglacial event yet dated and may have triggered subsequent deglacial events through eustatic sea level effects

chert occurrences in carbonate turbidites examples from the upper jurassic of the betic mountains southern spain in upper jurassic carbonate turbidites of the betic mountains southern spain chert occurs in three morphologies bedded chert nodular chert and mottled chert the last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock the three types of chert are formed by replacement of limestones and are associated with different calcareous facies turbidite packstones of saccocoma and peloids and turbidite lime mudstones of pelagic material contain bedded and nodular cherts the silicification textures are mainly micro and cryptocrystalline quartz with local chalcedonic quartz both length fast and length slow which is more common in the packstones only mottled chert is produced where calcareous breccia beds are silicified mottled chert consists of micro and cryptocrystalline quartz length slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz beds and nodules are the result of early diagenetic silicification with silica derived from the calcitization and dissolution of radiolarians and subordinately sponge spicules whereas mottled chert is the consequence of later silicification in a probably mg rich environment early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water a silica rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert

authigenic euhedral megaquartz crystals in a quaternary dolomite abstract autochthonous doubly terminated euhedral megaquartz crystals as well as overgrowths on detrital quartz grains are present throughout a gypsum and anhydrite bearing pleistocene sabkha dolomite accumulation from the arabian gulf the siliceous precipitates whose source was probably recycled biogenic silica formed within meters of the sediment surface these quaternary precipitates provide the first modern analog for euhedral megaquartz crystals a relatively common constituent in ancient evaporite bearing tidal flat carbonate strata in the quaternary dolomites studied megaquartz is present as individual crystals and clusters of crystals as well as overgrowths on detrital quartz grains individual crystals commonly attain lengths of 1 mm and many show shallow dolomite mold impressions on their crystal facies and or partly or completely engulf dolomite rhombohedra the common occurrence of dolomite molds within the quartz crystals indicates that precipitation of the quartz took place either concurrently with or more likely after dolomitization of the host carbonates additionally the oft cited association of authigenic megaquartz and evaporite minerals plus the existence of void spaces into which the quartz precipitated imply a causal relationship between dissolution of the evaporites and or dolomites and precipitation of the megaquartz thus these findings demonstrate that authigenic megaquartz crystals as well as overgrowths on detrital quartz can originate under near surface temperatures and pressures and very early in the diagenetic history of evaporite associated carbonate strata