reply to comment by s arai on primitive basaltic melts included in podiform chromites from the oman ophiolite by p schiano et al luboratoire de gpochimie cosmochimie cnrs ura 171x ipg paris et ufr sciences de la terre universitk paris vii 4 place jussieu f 75252 paris cedex 05 france luboratoire de minkalogie mu m national d histoire naturelle cnrs urh 736 61 rue de bujfon f 75005 paris france laboratoire pierre siie cea cnrs umr 9956 centre d etude nuclkaire de saclay f 91191 gij sur yvette france

magnetostratigraphy of the willwood formation bighorn basin wyoming new constraints on the location of paleocene eocene boundary abstract the lower eocene willwood formation in the bighorn basin of wyoming preserves a rich and diverse mammalian and floral record the paleomagnetic behavior of the sequence of floodplain paleosols of varying degrees of maturation ranges from excellent to poor we present a magnetostratigraphic section for a composite section near worland wyoming by using a set of strict criteria for interpreting the step wise alternating field and thermal demagnetization data of 266 samples from 90 sites throughout the composite section correlation to the geomagnetic reversal time scale was achieved by combining magnetostratigraphic and biostratigraphic data from this section from a section in the clark s fork basin in northern wyoming and from dsdp site 550 with the isotopic data determined on a tuff near the top of our section our correlation suggests that the bighorn basin composite section in the worland area spans from within chron c24r to near the top of chron c24n or from approximately 55 to 52 ma this correlation places the paleocene eocene boundary within the vicinity of the base of the section cryptochron c24r 6 of cande and kent is tentatively identified some 100 m above the base of the section the temporal framework provided here enables correlation of the mammalian biostratigraphy of the bighorn basin to other continental sequences as well as to marine records it also provides independent chronological information for the calculation of sediment accumulation rates to constrain soil maturation rates we exclude an age as young as 53 ma for the paleocene eocene boundary and support older ages as recommended in recent time scales the location of a tuff dated at 52 8 0 3 ma at the older boundary c24n 1 is consistent with the age of 52 5 ma estimated by cande and kent and inconsistent with that of 53 7 ma from harland et al

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

magnetic polarity stratigraphy and biostratigraphy of paleocene and lower eocene continental deposits clark s fork basin wyoming paleomagnetic samples were collected at 273 sites within a 2012 m continental sedimentary sequence in the clark s fork basin near powell wyoming the lower 1158 m is in the polecat bench fm while the upper 854 m is within the willwood fm although significant secondary components of magnetization were present alternating field demagnetization to 300 oe peak field revealed the polarity of the primary component of the natural remanent magnetism a well defined polarity zonation is observed the polecat bench fm is dominantly of reversed polarity but does contain two normal polarity zones the willwood fm in this section is entirely of reversed polarity abundant tiffanian clarkforkian and wasatchian faunas indicate that the age range of the sediments is from late paleocene through early eocene these age constraints along with the characteristic polarity sequence allow the magnetic polarity sequence to be correlated with the magnetic polarity time scale the section correlates with the magnetic polari

evidence for rapid climate change in north america during the latest paleocene thermal maximum oxygen isotope compositions of biogenic phosphate from the bighorn basin wyoming oxygen isotope records of cenozoic sea water temperatures indicate that a rapid warming event known as the latest paleocene thermal maximum lptm occurred during the otherwise gradual increase in world temperatures during the late paleocene and early eocene oxygen isotope analysis of the carbonate and phosphate components of hydroxyapatite found in mammalian tooth enamel and body scales of river dwelling fish from the bighorn basin in wyoming were made to investigate corresponding changes in the terrestrial climate a comparison of carbonate and phosphate isotope data from modern and fossil material indicates that some diagenetic alteration of the fossil material has occurred although systematically larger intra tooth ranges in the oxygen isotope composition of carbonate indicate that it is more likely to have been affected than phosphate carbonate and phosphate from the ecologically diverse mammals and fishes both record a shift to higher oxygen isotope ratios at the same time and of the same duration as the lptm these shifts reflect a change in the isotopic composition of regional precipitation which in turn provides the first evidence for continental climate change during the lptm assuming the present day relation between the oxygen isotope composition of precipitation and temperature applies to conditions in the past and that animal physiology and behavior is relatively invariant over time the isotopic shift is equivalent to an increase of surface temperature in western north america of several degrees this result is consistent with the magnitude of high latitude ocean warming and provides a basis for relating marine and terrestrial oxygen isotope records to records of terrestrial biotic change 1998 elsevier science b v all rights reserved

lower eocene alluvial paleosols willwood formation northwest wyoming u s a and their significance for paleoecology paleoclimatology and basin analysis abstract the lower eocene willwood formation of northwest wyoming is a 700 m thick accumulation of alluvial floodplain and channel mudstones and sandstones nearly all of which show paleopedogenic modifications pedogenesis of willwood sandstones is indicated by taproot and vertebrate and invertebrate bioturbation early local cementation by calcium carbonate and thin illuviation cutans on clastic grains pedogenesis in willwood mudstones is indicated by plant bioturbation insect and other invertebrate burrow casts and lebensspuren free iron aluminum and manganese mobilization including hydromorphic gleying sesquioxide and calcareous glaebule formation in lower parts of the solum presence of clay rich and organic carbon rich zones and well differentiated epipedons and albic and spodic horizons probable a horizons are also locally well developed occurrence of variegated paleosol units in thick floodplain mudstone deposits and their association with thin lenticular and unconnected fluvial sandstones in the willwood formation of the central and southeast bighorn basin suggest that these soils formed during times of rapid sediment accumulation the tabular geometry and lateral persistence of soil units as well as the absence of catenization indicate that willwood floodplains were broad and essentially featureless all willwood paleosols were developed on alluvial parent materials and are complex in that b horizons of younger paleosols were commonly superimposed upon and mask properties of suspected a and b horizons of the next older paleosols the soils appear to be wet varieties of the spodosol and entisol groups aquods and ferrods and aquents respectively though thick superposed and less mottled red purple and yellow paleosols resemble some ultisols most willwood paleosols resemble warm temperate to subtropical alluvial soils that form today under alternating wet and dry conditions and or fluctuating water tables the up section decrease in frequency of gley mottles increase in numerical proportion and thickness of red versus orange coloration and increase in abundance of calcrete glaebules indicate better drained soils and probably drier climate in late willwood time this drying is believed to be related to creation of rain shadows and spacing of rainfall but not necessarily decrease in absolute rainfall due to progressive tectonic structural elevation of the mountainous margins of the bighorn basin

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

structures in podiform chromite from the maqsad district sumail ophiolite oman about 15 chromite bodies have been recognized in the maqsad area of the oman ophiolite the occurrence in this area of three chromite bodies within the cumulate sequence must be integrated into the classification of cassard et al 1981 which presently explains only those pods lying in the uppermost mantle sequence plastically deformed harzburgites and dunites the occurrence of chromite bodies within the cumulates and the abundance of chromite in the maqsad area are related to the exceptional magmatic activity and the unusual plastic flow pattern particular to this area it was probably a feeding zone along the oceanic spreading center sitting on top of a mantle diapir

oxygen and hydrogen isotope variations among goethites α feooh and the determination of paleotemperatures abstract a plot of d vs 18o values from a diverse group of natural goethites defines a data array which is approximately parallel to the meteoric water line the 18o values range from 9 5 to 4 4 per mil while the corresponding d values vary from 202 to 94 per mil there is considerable scatter in the d vs 18o array which can be attributed primarily to different temperatures of mineral formation as determined previously yapp and pedley 1985 and confirmed in this study the goethite water d h fractionation factor is essentially independent of temperature over the range from 25 c to 145 c however the goethitewater 18 o 16 o fractionation factor does vary with temperature as indicated by initial goethite synthesis experiments conducted at 25 c 44 c and 62 c the synthetic goethite 18 g w values suggest that the natural goethites in this study formed at temperatures that may range from about 8 c to 66 c support for the validity of these approximate calculated temperatures is provided by a natural goethite sample from the atlantis ii deep in the red sea these results suggest that goethite can be used as a single mineral low temperature geothermometer perhaps more importantly goethite can be used in low temperature oxygen isotope mineral pair geothermometers such as chert goethite calcite goethite phosphate goethite etc

oxygen isotope systematics in a multiphase weathering system in haiti abstract physical and chemical partial dissolution techniques have been applied to a suite of young karst bauxite and laterite samples from the southern peninsula of haiti the 18o values have been obtained for nine mineral species and range from 1 0 for anatase rutile to 33 4 for authigenic quartz on one hand results for quartz calcite kaolinite gibbsite and boehmite compare favourably with 18o values predicted from accepted mineral water fractionation factors assuming modern temperatures 25 2 c and water 18o values 3 1 0 5 on the other hand the 18o values measured for anatase illite smectite chlorite smectite and some iron oxide samples do not compare favourably with predicted values departures from expected values for these minerals may be related to crystallization from water with a different 18o value than modern water an imprecise knowledge of some mineralwater fractionation factors under surficial conditions or nonattainment of isotopic equilibrium between mineral and water during formation partial dissolution techniques hold considerable promise for obtaining quantitative 18o values of individual minerals in single samples of complex fine grained regolith material

melt mineral fluid interactions in ultramafic nodules from alkaline lavas of mount etna sicily italy melt and fluid inclusion evidence abstract clinopyroxene phenocrysts and ultramafic nodules in the ancient alkaline lavas of mount etna contain melt and fluid inclusions which represent immiscible trapping of basaltic melt and co2 inclusions consist of glass vapor co2 hosted by clinopyroxene with kaersutite magnetite spinel and apatite daughter minerals type i and liquid co2 vapor co2 glass secondary inclusions type ii with highly variable fluid glass ratios electron microprobe investigations on type i melt inclusions indicate a glass of trachitic composition with extremely high chlorine contents 0 5 1 wt which we believe evolved from an initial melt composition of an alkali basalt homogenization temperatures th n 128 of type ii co2 glass inclusions display a bimodal fluid density distribution with high density 0 61 0 75 g cm3 co2 fluids in the nodules and low density 0 23 0 61 g cm3 co2 fluids in the phenocrysts a two step degassing process is proposed with high density immiscible co2 fluids formed at pressures of about 4 3 kbar the density distribution suggests interaction events between nodules ascending lavas and fluid phases with a peak around 3 5 kbar a second generation of lower density co2 is recorded around 2 8 kbar most likely prior to eruption

equability continentality and tertiary climate the crocodilian perspective the distributions of fossil crocodilians are used to examine north american continental paleoclimates for four time periods in the cenozoic control groups provide a qualitative means of examining the completeness of the record and thus the validity of crocodilian patterns by analogy with the range of the extant american alligator alligator mississippiensis the patterns imply that mid to high latitude continental interiors during the eocene and miocene did not undergo seasonal temperature extremes as great as those observed in such areas today this is compatible with the paleobotanical data during both the late oligocene and the pleistocene times of major glaciation crocodilians were restricted to more maritime localities

chronology of the wasatchian land mammal age early eocene magnetostratigraphic results from the mccullough peaks section northern bighorn basin wyoming a reply the mccullough peaks section of the willwood formation in the northern bighorn basin wyoming has produced a rich fossil record of early eocene mammals spanning much of the wasatchian land mammal age the wasatchian is an especially significant period in mammalian evolution since it marks the first appearance of several modern orders of mammals including perissodactyla artiodactyla and true primates magnetostratigraphic analysis of paleosol horizons in the mccullough peaks region is used to correlate wasatchian land mammal zones to the geomagnetic polarity time scale a total of 135 paleomagnetic samples were analyzed from 37 levels in a 1480 m section that ranges from wasatchian zone wa 0 earliest sandcouleean subage to the base of zone wa 7 earliest lostcabinian subage progressive thermal demagnetization of samples from red b type and mottled red gray ab type soil horizons provides the most reliable results the characteristic magnetization is carried by hematite with unblocking temperature

hematite and calcite coatings on fossil vertebrates abstract hematite coatings are common on vertebrate fossils from paleocene eocene paleosol deposits in the bighorn basin wyoming in general hematite coatings are found only on fossils and are limited to soils exhibiting hydromorphic features and moderate maturity petrographic and isotopic evidence suggests that hematite and micritic calcite formed at nearly the same time in a pedogenic environment whereas sparry calcite formed later at greater burial depths the parent material of paleosols is rich in iron supplying an ample source of iron for hematite formation decomposition of animal tissues around bones may enhance the weathering of iron bearing minerals in soils surrounding carcasses while the bones might provide favorable sites for iron accumulation the predominance of discrete smectite together with regional geothermal history suggests that burial temperatures have not exceeded 70 c hematite coatings on fossils can serve as a substrate for geochemical analysis in continental paleoclimatic research owing to their pedogenic origin abundance and resistance to diagenetic alteration

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

permo pennsylvanian palaeotemperatures from fe oxide and phyllosilicate δ18o values abstract the oxygen isotope composition of fossil roots that have been permineralized by hematite are presented from eight different stratigraphic levels spanning the upper pennsylvanian and lower permian strata of north central texas hematite 18o values range from 0 4 to 3 7 the most negative 18o values occur in the upper pennsylvanian strata and there is a progressive trend toward more positive 18o values upward through the lower permian strata this stratigraphic pattern is similar in magnitude and style to 18o values reported for penecontemporaneous authigenic palaeosol phyllosilicates and calcites suggesting that all three minerals record similar paragenetic histories that are probably attributed to temporal palaeoenvironmental changes across the late pennsylvanian and early permian landscapes palaeotemperature estimates based on paired 18o values between penecontemporaneous hematite and phyllosilicate samples suggest these minerals co precipitated at relatively low temperatures that are consistent with a supergene origin in a low latitude soil forming environment hematite phyllosilicate 18o pairs indicate 1 relatively low soil temperatures 24 3 c during deposition of the upper pennsylvanian strata followed by 2 a considerable rise in soil temperatures 35 37 3 c during deposition of the lowermost permian strata significantly d and 18o values of contemporaneous phyllosilicates provide single mineral palaeotemperature estimates that are analytically indistinguishable from temperature estimates based on hematite phyllosilicate oxygen isotope pairs the results between the two temperature proxy methods suggest that the inferred large temperature change across the upper pennsylvanian lower permian boundary might be taken seriously if real such a significant climate change would have undoubtedly had far reaching ecological effects within this region of pangaea notably there are important lithological and palaeobotanical changes such as disappearance of coal and coal swamp floras across the upper pennsylvanian early permian boundary of north central texas that may be consistent with major climatic change toward warmer conditions

stable isotope stratigraphy and paleoclimatology of the paleogene bighorn basin wyoming usa abstract climatic warming from the late paleocene into the early eocene had profound effects on atmospheric and marine circulation marine thermal gradients and benthic biota in addition marine carbon isotope values decreased substantially in both surface and deep waters because carbon is rapidly exchanged between reservoirs at the earth s surface such as marine surface water the atmosphere land plants and materials that obtain carbon from plants e g soil minerals and herbivores carbon isotope fluctuations provide time lines linking the marine and continental records we analyzed the carbon isotope composition of paleosol carbonates and mammalian tooth enamel from stratigraphic sections in northwestern wyoming carbon isotope correlation demonstrates that a short interval of extreme high latitude warming coincided precisely with the first appearance of several important modern mammalian orders in addition oxygen isotope analyses of fossils and paleosol carbonates provide information about climatic conditions on land in the paleogene we reconstructed the oxygen isotope composition of local meteoric water using biogenic minerals paleocene eocene meteoric water was significantly 18 o depleted indicating substantial loss of water vapor during its transport to the region soil temperature was calculated as a proxy for mean annual temperature assuming oxygen isotope equilibrium between soil carbonate and meteoric water calculated temperatures were plausible 10 25 c but highly variable prohibiting high resolution analysis of local temperature variations in response to global climatic warming

podiform chromitites of the tari misaka ultramafic complex southwestern japan as mantle melt interaction products petrological and geochemical data for chromian spinel from the tari misaka ultramafic complex southwest japan are discussed in this complex the spinels form a chromitite pod coated by a dunite envelope and are collectively enclosed by clinopyroxene bearing harzburgite chromitite and its dunite envelope are distinctly different from the surrounding harzburgite in terms of the chromian spinel chemistry the ti content is five to ten times higher in spinels of the enveloped chromitite than in those of the harzburgite although the cr ratio cr cr al the atomic ratio for both is almost constant or varies only slightly 0 4 0 5 this eliminates a comagmatic origin for the harzburgite dunite chromitite association spinel chemistry for both major and trace elements is almost the same in the chromitite and its dunite envelope except for the mg ratio and the co and mn contents which have a changed subsolidus as a function of the spinel mode thus the dunite and chromitite could be comagmatic but fractional crystallization of the dunite after or before chromitite deposition is not possible the chemistry of the chromian spinel and the mode of occurrence of the dunite chromitite association are consistent with an origin by interaction of an exotic melt with wall rock harzburgite the dunite may be of replacive origin the exotic melt would react with the harzburgite at the new contact to form dunite and a secondary si rich melt the melt thus formed could mix with a successive inflow of a more primitive melt the mixed melt would be expected to have chromite chromian spinel as the only liquidus phase precipitating chromitite next to the replacive dunite the dunite envelope podiform chromitite is most commonly formed in the mantle beneath island arcs the spread in cr ratios of spinel in podiform chromitites is almost comparable with that of spinel in primitive arc related magmas the presence of chromitite in the upper mantle beneath the japan arcs is consistent with this interpretation

eocene continental climates and latitudinal temperature gradients global climate during the mesozoic and early cenozoic is thought to have been warmer than at present but there is debate about winter temperatures paleontological data indicate mild temperatures even at high latitudes and in mid latitude continental interiors whereas computer simulations of continental paleoclimates produce winter temperatures closer to modern levels foliar physiognomy and floristic composition of 23 eocene floras from the interior of north america and australia indicate cold month means generally 2 c even where the mean annual temperature mat was

short term sediment accumulation rates determined from eocene alluvial paleosols a new method uses alluvial paleosols to calculate sediment accumulation rates for thin 25 m stratigraphic intervals and allows the reliable interpolation of ages for stratigraphic levels within a thick stratigraphic interval bounded by established dates sediment accumulation rates calculated for a 650 m composite section in the eocene willwood formation of wyoming span time intervals ranging from only 0 05 to 0 25 m y important sedimentologic changes coincide with changes in accumulation rate and indicate close and direct relations between the history of basin subsidence and depositional patterns

an assessment of isotopic equilibrium in goethites from a bog iron deposit and a lateritic regolith abstract hydrogen and oxygen isotope ratios have been measured in goethites from a lateritic regolith in the eastern amazon basin of brazil and a young bog iron deposit in new jersey usa the presence of exchangeable hydrogen and admixed minerals required the use of material balance calculations to determine relevant d and 18o values for the brazilian goethite d is 121 and 18o of 1 2 adjusted for al content these values yield a calculated temperature of formation of 24 3 c which is in good agreement with the modern average annual temperature in that locale goethite from the new jersey bog has a d of 134 and 18o of 0 9 its calculated temperature of formation 23 3 c resembles summer temperatures in southeastern new jersey bacterially mediated rates of precipitation of ferric hydroxide appear to be higher in the bog during the summer months crerar et al 1979 thus the temperature calculated for the new jersey goethite could indicate that the conversion of ferric hydroxide to goethite also occurs predominantly in the summer 13c values of the fe co3 oh component in goethite 21 7 for brazil and 19 3 for new jersey reflect the 13c values of the associated organic carbon 28 4 for brazil and 26 3 for the new jersey values of pco2 in the regolith 0 063 bar and bog 0 039 bar were calculated from the measured abundances of the fe co3 oh component the values of 13c and pco2 obtained from the goethites suggest that the ambient co2 in these systems originated primarily from oxidized organic matter in the low ph high pco2 waters characteristic of bogs and laterites isotopic equilibrium may be closely approached during the formation of goethite from ferric hydroxide ferrihydrite even if the initial precipitation of the ferric hydroxide precursor is bacterially mediated therefore meaningful paleotemperatures might commonly be calculated from stable isotope ratios of nonexchangeable oxygen and hydrogen in geothites from such environments

cool tropical temperatures shift the global δ18o t relationship an explanation for the ice core δ18o borehole thermometry conflict the discrepancy between central greenland borehole temperatures and the isotopic composition of last glacial maximum ice can be explained by a shift in the 5180 t relationship for the hydrological cycle linked to cooler tropical temperatures this concept is illustrated using a simple rayleigh distillation model an estimate for o a 5180 at lgm holocene of 0 37 o oc is determined with a simple graphical technique

formation of podiform chromitites by melt rock interaction in the upper mantle high cr and high al podiform chromitites are derived from melts formed by high and low degrees of partial melting in the upper mantle respectively the close association of the chromitites regardless of compositions with dunites and harzburgites can be explained by interaction between melts and harzburgites or depleted lherzolites in the upper mantle continuous reaction between melt and wallrock modifies the compositions of the magmas towards the precipitation of chromite alone under upper mantle conditions the modified magmas would have higher silica and lower magnesia and would be extracted into the crust or erupted leaving chromite pods in situ with dunite rinds that grade to harzburgites or lherzolites

rusty relics of earth history iron iii oxides isotopes and surficial environments abstract the two most common low temperature iron iii oxides on earth are goethite feooh and hematite fe2o3 the 18o values of natural goethites range from 15 5 to 3 3 whereas 18o values of low temperature hematites range from 16 7 to 4 7 plots of d against 18o for continental goethites are approximately parallel to the meteoric water line of craig h craig 1961 science 133 1702 3 this suggests that goethite water fractionation factors are systematic over a wide range of surficial environments and may indicate that isotopic equilibrium is commonly attained or closely approached several experimental or calculated mineral water oxygen isotope fractionation curves have been determined for both goethite and hematite although there is not yet a consensus on which of these curves best approximates isotopic fractionation in natural samples oxygen isotope measurements of both goethite and hematite have provided evidence of significant continental climate change on time scales that

experimental crystal growth in glass inclusions the possibilities and limits of the method glass inclusions trapped in bytownite phenocrysts from the ardoukoba eruption djibouti republic 7 november 1978 are used as an experimental environment to follow the evolution of crystallization and residual liquids in a magma with tholeiitic affinities the results are compared with those obtained from abyssal tholeiites by walker et al 1979 in near equilibrium crystallization conditions simultaneous crystallization of olivine and pyroxene is obtained in the enclosed silicate liquid as well as plagioclase as overgrowths on the cavity walls oxides only appear at lower temperatures the olivine is homogeneous and rather rare 5 in weight it forms at a temperature between 1 186 c 3 fo85 and 1 126 c fo66 the distribution coefficient of mg between the liquid and the olivine varies regularly with the temperature until titanomagnetite appears the augite is much more abundant 20 in weight of the original trapped melt and is characterized by a composition that varies from the centre to the edges of the crystals wo 43 5 36 2 en 43 8 41 7 fs 12 7 22 1 composition shows an unsatisfactory correlation with temperature and at most an fs enrichment may be noted with temperature decrease the composition of the plagioclase deposited on the cavity walls varies linearly with temperature from an 73 3 at 1 186 c to an 56 5 at 1 135 c in the case of contemporaneous crystallization of pl cpx ol and ox if experiments are carried out by undercooling as much as 150 c in relation to crystallization of the ferromagnesian minerals the composition of the plagioclase changes from an 78 8 at 1 160 c to an 52 8 at 1 025 c in the case of the pl ol cpx and ox crystallization the included liquids evolve from tholeiitic basalts to ferrobasalts in the same way as the lavas studied in the asal rift demange et al 1980 the advantages and limitations of the method are discussed

paleocene eocene climatic variation in western north america evidence from the δ18o of pedogenic hematite reconstruction of the pre pleistocene climate of continental interiors has been hampered by a lack of climatic proxies and limited temporal resolution we developed a new approach based on the oxygen isotope composition 18 o of hematite which occurs as a coating on fossil vertebrates and used this method to study climatic change across the paleocene eocene boundary in the bighorn basin wyoming hematite coatings form in soils or shallow sediments and therefore may be used to monitor near surface conditions hematite 18 o values are measured by combining selective leaching laser fluorination and mass balance calculation these values in conjunction with 18 o values for co occurring carbonates are used to assess the plausibility of published hematite water oxygen isotope fractionation relations though uncertainty remains as to the appropriate fractionation all are relatively insensitive to temperature variations thus differences in hematite 18 o values most likely reflect shifts in surface water 18 o values analysis of hematites spanning the paleocene eocene transition reveals a roughly 4 decrease in the 18 o of surface water in very early eocene time this episode probably reflects a cooling event perhaps as great as 6 c that started 0 7 m y after the terminal paleocene 13 c excursion which corresponds to a pulse of extreme marine warming following the cooling which lasted 0 6 m y temperatures rebounded to values close to those for late paleocene time the cooling episode coincides remarkably well with other indicators of environmental and climatic change from the basin such as mammalian turnover events and mean annual temperatures estimated from leaf physiognomy

laser oxygen isotope analysis of weathering goethite from the lateritic profile of yaou french guiana paleoweathering and paleoclimatic implications abstract oxygen isotopic compositions of weathering goethite pseudomorphs after pyrite in the lateritic profile of yaou french guiana were measured by laser fluorination the laterite at yaou is composed of a 25 m thick saprolite overlain by a 3 m thick latosol petrographic observations indicate that the pseudomorphic replacement of pyrite by goethite occurs at or near the weathering front and is complete the goethite pseudomorphs are essentially devoid of al and are progressively dissolved and partially replaced by a microcrystalline plasma of kaolinite and goethite in the upper horizons latosol of the profile the laser technique used in this study permitted the determination of the 18 o value of individual grains of goethite and the investigation of grain to grain variation within a single sample horizon as well as vertical variation with depth in the latosol the goethite 18 o values range from 0 8 to 3 2 and exhibit significant grain to grain and vertical variation this most likely reflects undetected intragrain contamination with microcrystalline kaolinite in the saprolite the goethite 18 o values are consistent ranging from 1 0 to 1 8 between 3 and 18 m depth and from 2 5 to 3 3 between 23 m depth and the weathering front at 28 m oxygen isotope compositions of present groundwater soil water and rainfall at yaou were also measured and compared with calculated isotopic compositions for goethite forming waters goethite pseudomorphs in the lower part of the saprolite are in isotopic equilibrium with present groundwater indicating that they may be forming today or that they formed in the recent past under climatic conditions similar to present conditions in contrast goethite pseudomorphs found higher in the profile are not in isotopic equilibrium with present water but with a water depleted in 18 o by 1 50 relative to present groundwater it is suggested that these low 18 o pseudomorphs are probably older and formed under different paleoclimatic conditions at a time when climatic regimes were possibly more monsoonal than today this is in agreement with models of climatic evolution derived from pedological considerations the results of this study suggest that goethite pseudomorphs in ancient saprolites may preserve their original o isotopic composition acquired at the weathering front and may therefore be used as indicators of changes in weathering climatic conditions during the evolution of a profile

the hydrogen and oxygen isotope geochemistry of pedogenic clay minerals principles and theoretical background abstract the 18 o 16 o and d h ratios of the structural oxygen and hydrogen of naturally occurring clay minerals carry information about the conditions under which the minerals formed clays generally form in isotopic equilibrium with the environment and hence have isotopic compositions that reflect the temperature of formation and the 18 o 16 o and d h ratios of water in the micro environment the isotopic signature of water in the weathering environment is a reflection of the climate once formed clays are resistant to isotopic alteration under surface and near surface conditions except when they undergo subsequent mineralogical alteration in some instances this alteration may be subtle as in the case of conversion of kaolinite with a high degree of crystalline order as measured by the hinckley index to kaolinite of low crystalline order in a pedogenic profile isotopic studies of pedogenic clays may be expected to yield more information about pedogenesis and paleoclimate when both 18 o 16 o and d h ratios of the clays are measured however few studies have incorporated the use of the two isotope ratios

the rapid heating defocused beam technique a co2 laser based method for highly precise and accurate determination of δ18o values of quartz abstract we describe a laser fluorination technique specifically for quartz that allows both high accuracy and high precision determination of oxygen isotope ratios regardless of grain size the rapid heating defocused beam technique rhd utilizes a defocused 32 w co 2 laser at full power which delivers 17 w 20 w mm 2 to the sample to quickly react quartz in contrast other recent laser fluorination studies use a tightly focused laser beam and slow heating i e sharp and kirschner 1995 fouillac and girard 1996 kirschner and sharp 1997 and find laser analysis of fine grained quartz to be inaccurate three quartz standards nbs 28 qz bra and qz cwru were analyzed using the rhd technique at the university of wisconsin to test for grain size effects rhd analyses show no correlation between grain size and 18 o values and are in excellent agreement with 18 o values obtained using conventional fluorination techniques additional analyses of the same quartz standards performed at university of wisconsin using slow heating and a focused beam yield 18 o values that are both less precise and significantly up to 0 8 lower than the accepted values attempts to use the rhd technique with a 20 w co 2 laser which delivers 8 w 10 w mm 2 to the sample at brgm were unsuccessful probably due to insufficient power density of the defocused beam we conclude that the rhd technique yields excellent accuracy and precision but that power densities of 15 20 w mm 2 may be necessary across a large part of the sample surface we recommend the use of lasers with at least 30 w of power

silicate and base metal sulfide inclusions in chromites from the maqsad area oman ophiolite gulf of oman a model for entrapment abstract al rich chromites from the maqsad area oman ophiolite have been studied to propose a genetic model for silicate and base metal sulfide bms inclusion entrapment samples were collected in both concordant and discordant nodular pods and in a peculiar stratiform ore chromite ore contains olivine clinopyroxene plagioclase as interstitial silicates regardless of the ore type the silicate inclusions are in decreasing order of abundance composed of ti and na rich phlogopite ti pargasite enstatite mg diopside olivine albite and secondary hydrothermal products chromite hosted bms are dominated by pentlandite heazlewoodite millerite and are nearly devoid of fe and cu sulfides the samples which come from concordant ores are very poor in mineral inclusions a consequence of the strain induced grain boundary migration and recrystallization which they suffered during plastic deformation in contrast the nodular and stratiform samples which escaped post magmatic deformation and densification are inclusion rich the inclusions display negative crystal shape following the chromite symmetry if present in nodular samples they may be arranged as coronas which mimic the shape of the host chromite grains all these textural features are considered to be evidence for mineral inclusion entrapment during magmatic precipitation of chromite by reference to a previous dynamic model of chromite ore genesis we consider that the nodules of chromite crystallized from basaltic magma were flowing inside a narrow cavity in which a non turbulent convective system was established we deduce from the presence of several inclusion coronas or patches in the largest nodules and their aabsence in the smallest that the inclusions were trapped preferentially at the point in which the primitive magma flow meets the convective current the peculiar mineralogy of mineral inclusions dominated by hydroxyl sodic phases indicates a complex interaction at the magmatic stage between the chromite precipitating magma and an unrelated aqueous fluid phase this fluid which was sodium and sulfur bearing travelled through the top of the mantle sequence and the basis of cumulate rocks of the ophiolite it could have been injected at certain stages of its evolution in the same conduits as the chromite precipitating magma locally giving rise to volatile enriched differentiated melts the latter have been sealed together with the liquidus phases of magma olivine clinopyroxene orthopyroxene and plagioclase as inclusions during chromite growth and or dissolution olivine and pyroxenes have partly reacted with the trapped volatile rich melt producing phlogopite and pargasite several hundred degrees below trapping temperatures bms have formed through sulfidization reactions with enclosed silicates and or host chromite

early eocene biotic and climatic change in interior western north america imprecise correlation of the marine and terrestrial fossil records has been a major obstacle to understanding migration and extinction of continental biotas and early cenozoic climate change new sup 40 ar sup 39 ar data from the willwood formation in the bighorn basin of wyoming establish an age of 52 8 plus minus 0 3 ma for earliest lostcabinian late wasatchian faunas and coeval early eocene floras strata just beneath earliest wasatchian faunas can be correlated with the np9 np10 boundary in marine sedimentary units which has an interpolated age of approximately 55 7 ma this new information allows the authors to estimate the durations of the wasatchian approximately 5 m y and the lostcabinian approximately 2 m y and shows that the continental biotas are coeval with the acme of cenozoic warmth inferred from delta sup 18 o measurements of foraminifera from 58 to 50 ma paleoclimate in the continental interior at about 45 degree n was warm and equable but patterns of temperature change inferred from continental floras do not track precisely the marine delta sup 18 o record

apatite fission track evidence for laramide and post laramide uplift and anomalous thermal regime at the beartooth overthrust montana wyoming the crystalline rocks of the beartooth overthrust in southwest montana and northwest wyoming were uplifted and thrust over the northwest margin of the bighorn basin during the laramide orogeny the timing and geometry of that event is well documented in the synorogenic paleocene and early eocene sedimentary rocks that were deposited ahead of the rising beartooth basement block and partially over ridden as the block advanced the absence of post laramide sedimentary rocks on the block and in the adjacent bighorn basin has however precluded the reconstruction of the post orogenic tectonothermal history of this region we report here the results of fission track analysis of apatite from the three components of the beartooth over thrust with the aim of reconstructing the laramide and post laramide tectono thermal history of the southeast margin of the beartooth mountains apatite fission track ages and corresponding horizontal confined track length distributions hctlds from precambrian basement rocks constituting the upper plate of the beartooth overthrust indicate that from 7 to 12 km of uplift of the red lodge corner of the beartooth block has occurred since early paleocene time this amount of uplift occurred in two stages with an intervening mid tertiary period of tectonic quiescence the latter was a period of either 1 oligocene and or miocene deposition or 2 tectonic quiescence uplift of 4 to 8 km occurred during the first phase of cooling which lasted from early paleocene time 61 ma to early eocene time 52 ma during the second phase which began in late miocene early pliocene time and continues to the present day about 4 km of uplift occurred our fission track data suggest that the thermal regime in rocks above the beartooth overthrust was relatively stable during tertiary time the maximum geothermal gradient permitted by model thermal histories generated from our observed fission track data during post laramide time is 17 c km this value is the same as that of the present day geothermal gradient measured in the amoco beartooth well and suggests that a low geothermal gradient prevailed throughout tertiary time apatite fission track ages and hctlds from jurassic and cretaceous sedimentary rocks beneath the beartooth overthrust indicate that these rocks have remained 10 20 c cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since 61 ma an observation that is not consistent with the stratigraphic position of these rocks we interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks this ground water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the beartooth well shear zone rocks experienced a higher temperature during cenozoic time than did rocks of the upper and lower plates this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust

stable carbon and oxygen isotopes of pedogenic carbonates ajo mountains southern arizona implications for paleoenvironmental change abstract in this study we compare the stable isotope composition of late holocene pedogenic carbonate from a late holocene soil developed on the pediment below the ajo mountains in southern arizona usa with measurements of the 18 o of the soil water throughout the seasonal cycle and with 18 o and 13 c measurements on older dated soils the comparison with the soil water composition shows that the pedogenic carbonate is forming in equilibrium with highly evaporated soil water at the highest summer temperatures the 13 c of the carbonate apparently reflects equilibrium with co 2 of an isotopic composition consistent with the present mixture of c3 desert shrubs and cam cactus comparison with glacial period soil carbonates shows that there has been little change in the 18 o but that the 13 c was about 4 5 heavier independent evidence indicates that glacial period summer temperatures were probably much cooler and that the 18 o of precipitation was also lighter the increase in the carbonate water fractionation factor with decreasing temperature probably counteracted the decrease in the 18 o of precipitation producing little net change in the 18 o of the soil carbonate we attribute the decrease in the 13 c to replacement of a glacial period c4 grassland by holocene c3 cam desert shrubs and succulents this replacement was probably in response to an increase in temperature and reduction in summer precipitation at the end of the last glacial period the 13 c of older soil carbonates indicates that landscape was dominated by grassland for most of the past 700 ka and that the present vegetation likely represents a relatively brief anomaly

equable climates during the early eocene significance of regional paleogeography for north american climate the character of continental interior paleoclimate at mid latitudes especially the aspect of temperature has been a major source of debate in modeling and paleontologic communities over the past few years a recent climate modeling study provides new insight into the issue of climatic conditions of early eocene north america model cases with six times the present level of co 2 or with doubled present co 2 and a large lake in western north america produced results most similar to proxy paleoclimate interpretations it is significant that the north american continental interior climate responded as strongly to the existence of the lake as to the atmospheric co 2 level the large lake deflects the winter freeze line poleward of the region containing most paleoclimate data site locations producing above freezing winter temperatures and providing a possible solution to the minimum temperature difference that exists between models and data the effect of the lake upon regional climate is significant and proves to be critical to reproducing the early eocene climate of north america

stable isotope record in mollusca and pedogenic carbonate from late pliocene soils of central italy abstract the late pliocene younger than 2 5 ma alluvial fan succession of toppetti quarry near todi umbria central italy includes many palaeosols pedogenic carbonate concretions and mollusc remains were separated from 73 palaeosols collected along 150 m of exposure and analysed for their oxygen and carbon stable isotope content 13c and 18o of the pedogenic carbonate and land snail shells suggest that no dramatic change in local climate took place values of the pedogenic carbonates are similar to other data reported for the mediterranean region 18o data suggest a mean annual temperature and or 18o similar to modern climatic conditions carbon isotope data from both land snails and pedogenic carbonate are consistent with a prevailing type c3 vegetation a mixture of forest and c3 grassland sustained by a mean annual precipitation between 300 and 1000 mm mainly in the cool periods of the year seems to be the most likely scenario the limited variation shown by the isotope composition of the biogenic and inorganically precipitated carbonate indicates that the environmental variations are referable mainly to change of humidity rather than temperature

tectonic evolution of the alpujarride complex betic cordillera southern spain abstract the alpujarride complex belonging to the internal zones of the betic cordillera is an e w elongated domain of metamorphic nappes which extends over more than 400 km in southern spain the alpujarride complex suffered two shearing episodes defined by different kinematic and physical conditions the first one involved ductile thrusting from the south west towards the north east related to the regional metamorphism of the alpujarride nappes this process led to crustal thickening related to which the hot thrusting of the ronda peridotites occurred the second shearing episode directed northwards occurred after the metamorphism it corresponds to a deformation in the brittle ductile transition that led to the thinning of the crust thickened in the previous episode previous tectonic models of the betic cordillera have only considered the northwards shearing which reflect the n s convergence of europe and africa from the oligocene onwards the present work clearly points to the existence of previous episodes of ductile thrusting towards the north east that reflect an earlier process of continental collision and that in future should be considered in tectonic models of the betic orogen

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

climate and groundwater recharge during the last glaciation in an ice covered region a multitracer study of a small aquifer in northern switzerland indicates that the atmosphere in central europe cooled by at least 5 c during the last glacial period the relation between oxygen isotope ratios 18o and recharge temperatures reconstructed for this period is similar to the present day one if a shift in the 18o value of the oceans during the ice age is taken into account this similarity suggests that the present day 18o temperature relation can be used to reconstruct paleoclimate conditions in northern switzerland a gap in calculated groundwater age between about 17 000 and 25 000 years before the present indicates that during the last glacial maximum local groundwater recharge was prevented by overlying glaciers

vertebrate fossil bearing paleosol units willwood formation lower eocene northwest wyoming u s a implications for taphonomy biostratigraphy and assemblage analysis abstract in the lower 300 m of the fluvial willwood formation of northwest wyoming most large concentrations of vertebrate fossils consist of disarticulated and broken skeletal remains that occur in widespread tubular thin 2 cm 1 m greenish and bluish gray mudstones that are low in percentage of free iron aluminum and manganese relatively high in percentages of organic carbon and often show evidence of clay eluviation these units lie atop sesquioxide rich sometimes clay illuviated variegated unit red yellow purple and mottled variants and are relicts of the a horizons of podzolic spodosols aquods the wide distribution of bone fragments the numerical proportions of bone elements weathering of the fragments and biogenic evidence indicate the vertebrate remains accumulated gradually as litter on the surface of the soils and became incorporated into the a horizon the bones were disarticulated broken and otherwise disturbed by the action of scavengers prior to burial occurrence of abundant vertebrate remains in discrete readily identifiable and widespread paleosol units makes these units valuable biostratigraphic markers zonules faunules evidence of geologically short term lag accumulation of the bones is important to assemblage analysis the paleosol concentrations result from attritional mortalities that more closely reflect composition of the life assemblage than do remains concentrated by fluvial transport or predators

isotopic evidence for shifts in atmospheric circulation patterns during the late quaternary in mid north america wyoming is now at the eastern margin of westerlies originating in the pacific but in the pleistocene appears to have received moisture from elsewhere possibly the gulf of mexico oxygen isotope ratios of pedogenic carbonate in postglacial terraces correspond to ratios in equilibrium with present meteoric waters which show a strong relation to precipitation seasonality and storm sources in contrast the oxygen isotope ratios of all pre holocene soils are significantly more positive even though the carbon isotope composition of coexisting organic matter suggests that the carbonate formed in temperatures cooler than today the oxygen isotope ratios of paleowaters in mid north america appear to be more useful for identifying past storm sources than for estimating paleotemperatures

late paleocene early eocene climate changes in southwestern wyoming paleobotanical analysis the warmest global temperatures of the cenozoic era occurred in early eocene time following a warming trend that started in late paleocene time the greater green river basin of southwestern wyoming is one of the best areas in the rocky mountains for paleobotanical investigation of the paleocene eocene climatic transition intensive sampling has resulted in the recovery of an estimated 189 species of plant macrofossils from the tiffanian clarkforkian wasatchian and bridgerian land mammal ages the leaf morphologies and taxonomic affinities of these fossils were used in combination with other indicators to evaluate paleocene eocene climates following cool humid conditions in the tiffanian the clarkforkian was humid and subtropical and several plant families with modern tropical affinities appeared however as in the tiffanian clarkforkian floras had low diversity and were dominated by a single species in the birch family mean annual temperature mat rose from an estimated 12 c in the tiffanian to 19 c in the clarkforkian while mean annual precipitation map for the tiffanian and clarkforkian is estimated to have been 130 150 cm little fossil plant material is preserved from the latest clarkforkian or the earliest wasatchian which is thought to have contained an interval of cooling and drying followed by renewed warming by the middle wasatchian the time of the cenozoic thermal maximum the inferred mat was about 21 c and the map was near 140 cm a second influx of plant families with tropical affinities appeared in the area and diversity increased significantly but most plant families known from the clarkforkian persisted species turnover from the clarkforkian to the wasatchian was greater than 80 a second turnover of more than 80 of species but not families from the wasatchian to the early bridgerian accompanied drying and increased seasonality of precipitation the early bridgerian mat is inferred to have been near 20 c and the map to have been about 80 cm except for the tiffanian and possibly portions of the early wasatchian paleoclimates during the study interval were predominantly frost free although the moderating influence of the green river lake system has been suggested as a possible explanation for mild eocene winters in wyoming this study shows that virtually frostfree climates existed in the area prior to and independent of significant lake development

the chromite deposits of the troodos complex cyprus evidence for the role of a fluid phase accompanying chromite formation the troodos ophiolite of cyprus hosts chromite deposits both as podiform bodies within the basal harzburgite and as lesser layers and schlieren within the overlying dunites and clinopyroxene dunites of the metacumulate sequence all chromitite types have been studied from both field and mineralogical aspects with particular emphasis on the chemistry of the ubiquitous silicate inclusions within chromite grains two distinct inclusion assemblages have been documented chromites from podiform bodies contain abundant hornblende serpentine ortho and clinopyroxene and lesser olivine while those from the metacumulates host pargasite albite serpentine and rare pyroxenes this implies that no single silicate inclusion assemblage in chromite can be considered typically ophiolitic the presence of an alkalirich fluid phase during chromite crystallization as evidenced by the unusual mineralogy of these inclusions has already been widely discussed in the literature however the marked stratigraphic control of inclusion types in chromite as reported here enables more complete modelling of the evolution of this phase to be made