ages of some intrusive rocks of southwestern maine u s a whole rock rb sr and u pb zircon age measurements on intrusive rocks in southwestern maine indicate igneous activity at 400 340 and 320 ma these plutonic rocks were emplaced into deformed ordovi

global distribution of beryllium isotopes in deep ocean water as derived from femn crusts the direct measurement of the ratio of cosmogenic 10be t12 1 5 ma to stable terrigenously sourced 9be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea in this study a sims based technique has been used to determine the 10be9be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans 10be9be ratios time corrected for radioactive decay of cosmogenic 10be using 234u238u are in good agreement with ams measurements of modern deep seawater ratios are relatively low in the north and equatorial atlantic samples 0 4 0 5 10 7 in the southwest atlantic ratios increase up to 1 10 7 they vary between 0 7 and 1 0 10 7 in indian ocean samples and have a near constant value of 1 1 0 2 10 7 for all pacific samples if the residence time of 10be 10be in deep water is constant globally then the observed variations in 10be9be ratios could be caused by accumulation of 10be in deep water as it flows and ages along the conveyor following a transient depletion upon its formation in the northern atlantic in this view both 10be and 9be reach local steady state concentration in pacific deep water and the global 10be 600 a an alternative possibility is that the be isotope abundances are controlled by local scavenging for this scenario 10be would vary according to local particle concentration and would 600 a in the central pacific but 10be 230 a in the atlantic mass balance considerations indicate that hydrothermal additions of 9be to the oceans are negligible and that the dissolved riverine source is also small furthermore aeolian dust input of 9be appears insufficient to provide the dissolved 9be inventory the dissolution of only a small proportion 2 of river derived particulates could in principle supply the observed seawater 9be content if true ocean margins would be the sites for 9be addition due to the particle reactive nature of be these would also be the primary sites of be removal a possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10be9be and establishment of a relatively constant 9be concentration as 10be 600 a is less than the apparent age of deep water in the pacific 1500 a the pacific record of 10be9be is not expected to show secular variations due to changes in deep water flow despite the large variations in 10be9be between different water masses because of this insensitivity to deep water flow however it is suggested that the 10be9be ratio determined in the authigenic phase of marine sediments or hydrogenetic precipitates should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales

exchange of neodymium and its isotopes between seawater and small and large particles in the sargasso sea the concentrations and isotopic ratios of nd were measured in small 1 53 m and large 53 m in situ filtered particles sediment trap material sediment and aerosol samples collected during oligotrophic and productive conditions in the sargasso sea atmospheric input is the main source of exogeneous nd into the sargasso sea based on ndal ratios in trapped material we suggest that up to 50 of atmospheric nd could dissolve at the air sea interface the concentration and isotopic composition of nd vary with both depth and the particle size neodymium concentrations of small and large particles increase with depth from 2 9 12 g g and 5 8 11 8 g g respectively at 40 m the nd signature of large particles is significantly more negative nd 0 13 1 0 2 than that of the small ones nd 0 11 7 0 3 and the particles filtered deeper below 237 m the nd 0 values for small particles match the seawater whereas the nd 0 values of large particles remain less radiogenic these differences are related to the different behaviors of the particulate fractions small particles apparently exchange nd with seawater and then sink rapidly to depth as biologically formed aggregates large particles these aggregates isolate the small particles slowing the exchange with the water to produce different signatures between large particles and seawater since these exchange processes modify the particulate nd signature the shallow radiogenic nd 0 signature observed in the surface and subsurface waters in the nw atlantic is not due to external contributions and is likely of advective origin at depth below 800 m the magnitude of the aggregation disaggregation process varies significantly with season the fraction of small particles repackaged into large aggregates can vary from 0 8 to 80 100 in productive and oligotrophic conditions respectively this seasonal variation may correspond to sinking particles of different zooplanktonic origin vertical fluxes of nd into and out of the sargasso sea as well as exchange fluxes in the water column are examined about 50 of the particulate flux from the surface is remineralized at depth a net scavenging flux of 0 8 10 4 g m2 y is required this value corresponds to a scavenging nd residence time of 1900 y considerably longer than previous estimates of global oceanic residence time of nd on the order of 400 y

large scale stable isotope alteration around the hydrothermal carbonate replacement cinco de mayo zn ag deposit mexico carbonate hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos which inhibit exploration targeting in contrast hydrothermal modification of the country rock s stable isotope composition usually extends far beyond the limited visible alteration hence stable isotope studies should be an effective tool to aid exploration for carbonate hosted deposits here we present new insight into the development of a large stable isotope alteration halo based on 910 o and c isotope analyses of carbonate veins and hydrothermally altered limestone hosting the cinco de mayo pb zn ag au cu carbonate replacement deposit crd in chihuahua mexico our results demonstrate that stable isotope alteration is consistent with reactive magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems synmineralization veins are texturally and isotopically distinct from those formed during pre and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow this appears to be a promising exploration vectoring tool downhole variations in wall rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths furthermore wall rock 18ovsmow systematically decreases toward mineralization from 23 to 17 over a distance of 10 km providing another vectoring tool the extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system which may be expected to scale with the mineral endowment this suggests that constraining the size shape and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit

distribution and sources of pre anthropogenic lead isotopes in deep ocean water from fe mn crusts abstract the lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead here the global distribution of lead isotopes in deep ocean water is presented as derived from dated ca 100 ka surface layers of hydrogenetic fe mn crusts the results indicate that the radiogenic lead in north atlantic deep water is probably supplied from the continents by river particulates and that lead in pacific deep water is similar to that characteristic of island and continental volcanic arcs despite a short residence time in deep water 80 100 a the isotopes of lead appear to be exceedingly well mixed in the pacific basin there is no evidence for the import of north atlantic deep water derived lead into the pacific ocean nor into the north indian ocean this implies that the short residence time of lead in deep water prohibits advection over such long distances consequently any climate induced changes in deep water flow are not expected to result in major changes in the seawater pb isotope record of the pacific ocean

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

response of deep ocean circulation to initiation of northern hemisphere glaciation 3 2 ma carbon isotopic records from benthic foraminifera are used to map patterns of deep ocean circulation between 3 and 2 million years ago the interval when significant northern hemisphere glaciation began the 18o and 13c data from four atlantic sites 552 607 610 and 704 and one pacific site 677 show that global cooling over this interval was associated with increased suppression of north atlantic deep water nadw formation however the relative strength of nadw production was always greater than is observed during late pleistocene glaciations when extreme decreases in nadw are observed in the deep north atlantic our data indicate that an increase in the equator to pole temperature gradient associated with the onset of northern hemisphere glaciation did not intensify deepwater production in the north atlantic but rather the opposite occurred this is not unexpected as it is the warm high salinity characteristic rather than the low temperature of thermocline waters that is critical to the deepwater formation process in this region today

a lead isotopic study of circum antarctic manganese nodules abstract lead isotope ratios in manganese nodules from the circum antarctic ocean display systematic geographical variations eastward in the direction of current movement from south of australia they progressively decrease in the pacific sector and into the western atlantic sector until 30 w from this longitude lead isotope ratios increase in the eastern atlantic sector and reach their highest values in the indian sector near 110 e south of western australia they abruptly decrease these two lead isotope domains correspond closely to the circum antarctic neodymium isotope domains previously recognized by albarede and goldstein 1992 however the intraocean isotopic variations are much clearer for pb than nd reflecting the shorter oceanic residence time of pb 102 y compared with nd 103 y and confirming that pb is a much more sensitive marine tracer throughout the circum antarctic ocean the lead isotopic changes follow the pattern of present day oceanic circulation the pb shows an overall continental signature which is strongest in the indian sector the most probable candidate as the source of the continental signal is north atlantic deep water nadw which enters the circum antarctic in the south atlantic as nadw mixes with circumpolar water lead isotope ratios increase eastward in the indian sector reflecting progressive loss of the pacific derived pb signal the abrupt decrease of lead isotope ratios between the indian and pacific sectors appears to be a consequence of addition of pacific seawater through the pacific lead isotope ratios gradually decrease eastward approaching values of east pacific rise metalliferous sediments ocean ridge basalts and south pacific mn nodules this trend results from an increasing influence of pacific ocean water containing a component derived from pacific volcanism the pacific signature persists into the argentine basin in the southwest atlantic ocean reflecting significant input of water from the drake passage northward into the atlantic the pacific pb component is seen in the atlantic and indian oceans near southern africa as well as in the atlantic and indian sectors of the circum antarctic lead and neodymium isotopes correlate in the atlantic indian sectors but not in the pacific sector reflecting differences in residence times and modes of water mass mixing the renewal of circum antarctic water occurs on a timescale which is short compared to the residence time of nd but long compared to pb as a result dissolved pb is subject to considerable changes over the timescale of circumpolar current movement while nd is less easily affected nadw is added locally in the south atlantic and moves eastward as a water mass as nadw mixes with circumpolar water cpw through the indian sector changes affecting nd pb in nadw and cpw occur in tandem and neodymium and lead isotopes are correlated through the pacific sector gradual addition of pacific seawater occurs over a large region and nd pb relationships are not coupled because simple isotopic mixing systematics are not conserved there neodymium and lead isotopes vary independently the consistency of lead isotope variations with present day ocean bottom water circulation is remarkable considering that the mn nodules as sampled reflect the integrated flux of pb over 105 106 y the oceanic mixing time is 103 y the residence time for pb is 102 y and the circulation time of the circumantarctic ocean is 30 y processes occurring over such different timescales can show such a relationship only if the system is stable over the longest time interval thus on average element sources and ocean circulation through the pleistocene have been similar to the present day despite significant perturbations during glacial periods

a short circuit in thermohaline circulation a cause for northern hemisphere glaciation the cause of northern hemisphere glaciation about 3 million years ago remains uncertain closing the panamanian isthmus increased thermohaline circulation and enhanced moisture supply to high latitudes but the accompanying heat would have inhibited ice growth one possible solution is that enhanced moisture transported to eurasia also enhanced freshwater delivery to the arctic via siberian rivers freshwater input to the arctic would facilitate sea ice formation increase the albedo and isolate the high heat capacity of the ocean from the atmosphere it would also act as a negative feedback on the efficiency of the conveyor belt heat pump

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

210pb scavenging in the north atlantic and north pacific oceans the radionuclide210pb shows significant geographic variations in the extent of its removal from the open ocean water column this texture of scavenging is defined by mapping 1 the integrated deficiency of210pb in the water column relative to its supply from the atmosphere and from in situ decay of dissolved226ra and 2 inventories of excess210pb in deep sea sediments the ratio of210pb deficiency to its supply termed the scavenging effectiveness is 20 in the north equatorial pacific and 50 in the north atlantic this variation is related to the combined effects of uptake of210pb onto sinking particles and lateral transport of210pb to areas of more intense removal sediment inventories of excess210pb normalized to the210pb deficiency in the overlying water column permit evaluation of the relative importance of these effects in the north equatorial pacific virtually all of the210pb removed from the water column is present in the underlying sediments but in the mid latitude north atlantic the sediments comprise only about 50 of the210pb removed the deficiencies of210pb in the mid latitude north atlantic sediments south of 50 n are qualitatively offset by surpluses in high latitude sediments north of 50 n higher primary productivity and new production in the surface waters of the high latitude north atlantic and north equatorial pacific relative to the oligotrophic central north atlantic may account for the greater fluxes of210pb to bottom sediments in those areas

closure of the central american isthmus and its effect on deep water formation in the north atlantic modern ocean thermohaline driven circulation influences global climate by transporting heat to high latitudes1 2 and by affecting the exchange of co2 between ocean and atmosphere3 north atlantic deep water nadw plays a key role in this circulation and quaternary climate cycles have been linked to changes in nadw flow4 general circulation model simulations indicate that before closure some 3 4 million years ago of the central american isthmus the narrow strip of land linking north and south america the direct flow of low salinity water from the pacific to the atlantic ocean would have led to a smaller nadw flow5 6 sedimentation patterns7 and nutrient proxies8 11 support these model results by indicating an increase in nadw flow around the time of isthmus closure but these records do not allow changes in different nadw sources to be distinguished and the overall effect of closure on global ocean circulation is poorly known here we present nd pb and sr isotope records preserved by a hydrogenous ferromanganese crust from the nadw flow path in the western north atlantic ocean these records indicate that the isotopic signal associated with nadw strengthened around 3 4 million years ago showing that deep water that formed in the labrador sea made a gradually increasing contribution to nadw flow these data taken together with those from the central pacific ocean12 indicate an increased nadw flow since isthmus closure and suggest that the closure established today s general pattern of ocean circulation

60 myr records of major elements and pb nd isotopes from hydrogenous ferromanganese crusts reconstruction of seawater paleochemistry we compare the time series of major element geochemical and pb and nd isotopic composition obtained for seven hydrogenous ferromanganese crusts from the atlantic indian and pacific oceans which cover the last 60 myr average crust growth rates and age depth relationships were determined directly for the last about 10 myr using be 10 be 9 profiles in the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial be 10 be 9 ratios due to the lack of additional information co contents have also been used previously to estimate growth rates in co rich pacific and atlantic seamount crusts puteanus and halbach 1988 a comparison of be 10 be 9 and co based dating of three co rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated be 10 be 9 based growth rates back to 60 ma our data show that the flux of co into co poor crusts has been considerably lower the relationship between growth rate and co content for the co poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits manheim 1986 the results suggest that the co content provides detailed information on the growth history of ferromanganese crusts particularly prior to 10 12 ma where the be 10 based method is not applicable the distributions of pb and nd isotopes in the deep oceans over the last 60 myr are expected to be controlled by two main factors a variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and b variability of supply rates or provenance of detrital material delivered to the ocean linked to climate change glaciations or major tectonic uplift the major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust consequently any interpretation of time series of major element concentrations of a single crust in terms of paleoceanograghic variations must be considered with caution this is because local processes appear to have dominated over more basin wide paleoceanographic effects in this study co is the only element which shows a relationship to pb and nd isotopes in pacific crusts a possible link to changes of pacific deep water properties associated with an enhanced northward advection of antarctic bottom water from about 14 ma is consistent with the pb but not with the nd isotopic results the self consistent profiles of the pb and nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts including phosphatization events have been insignificant for particle reactive elements such as pb be and nd isotope time series of pb and nd show no systematic relationships with major element contents of the crusts which supports their use as tracers of paleo seawater isotopic composition

10 be dating of a manganese crust from central north pacific and implications for ocean palaeocirculation one of the most promising applications of the new 10be detection technique using accelerator mass spectrometry is the determination of growth rates of mn nodules and crusts1 5 because the half life of cosmogenic 10be 1 5 myr is of the same order as the time needed for a few millimetres of nodule material to grow 10be is a useful tool for unravelling such evolution during the late tertiary when most nodules are assumed to have started their growth6 mass spectrometric measurement of a 10be profile across the manganese crust va 13 2 from the central pacific using a tandem van de gr a ff accelerator yields growth rates of 2 7 and 4 8 mm myr 1 for the layers of the crust accumulated between recent and 6 myr bp and between 6 and 11 myr respectively using these measurements as well as 230th dating we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them these ages are 0 12 2 9 3 4 5 7 6 7 7 9 10 12 and 13 16 myr bp all of these boundaries apparently coincide with the times reported for quaternary and late tertiary palaeoclimatic events suggesting that the crust growth has been strongly influenced by palaeoclimate

oxygen isotopic compositions of the late orogenic granites in the southern piedmont of the appalachian mountains u s a and their relationship to subcrustal structures and lithologies abstract data were acquired from 143 whole rock samples from 20 late orogenic post metamorphic hercynian age granitic plutons from the piedmont of the southern appalachians principally in georgia and south carolina these plutons exhibit a regional gradient in oxygen isotopic compositions in which the granites confined to the inner piedmont to the northwest are 18 o enriched 11 4 to 7 9 whereas those toward the southeast within the charlotte slate and portions of the kiokee belts have distinctly lower 18 o 16 o compositions 8 2 to 5 5 one body that lies along the southeastern edge of the piedmont in south carolina however appears to be anomalously 18 o enriched 8 9 most plutons display 18 o 16 o variations of 18 o enriched plutons occupy areas characterized by negative anomalies whereas low 18 o bodies are invariably restricted to regions of positive anomalies 2 87 sr 86 sr data where granites with 18 o values 18 o enriched plutons 9 have ratios 0 710 3 contrasting chemical and accessory mineral compositions in which many 18 o depleted granites have a number of i type characteristics whereas several of the most 18 o enriched plutons exhibit a number of s type features it can be inferred from these data that the 18 o enriched granites were formed from continental crustal protoliths that underlie much of the inner piedmont and portions of the kiokee belt whereas the low 18 o plutons were derived from more mafic sources beneath the charlotte slate and portions of the kiokee belt the overall correspondence between the regional 18 o 16 o patterns exhibited by the granites and gravity data implies that these grantes may be essentially rooted to their protoliths in turn suggesting that the large scale translational movement recently proposed for the southern piedmont may have occurred prior to intrusion of these granites 320 m y ago

the enasen gold deposit central sweden stable isotope analyses of quartz sulphides and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the palaeoproterozoic enasen gold deposit reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks however oxygen isotope geothermometry on a coexisting quartz magnetite pair gave a minimum temperature for peak metamorphism of around 650 c which agrees with fe mg geothermometry this suggests that grain scale equilibrium is achieved the variation in oxygen isotope ratios 18o 7 3 10 5 on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher 18o values in more superficial parts temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit it is suggested that alteration silicification and mineralization at the enasen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200 250 c and that the hydrothermal fluids consisted of meteoric and magmatic water a tentative reconstruction of the fossil hydrothermal system is presented sulphur isotope ratios of sulphides from the fold bearing quartz sillimanite gneiss gave 34s values close to zero indicating a magmatic source of the sulphur

strontium neodymium and lead isotope variations of authigenic and silicate sediment components from the late cenozoic arctic ocean implications for sediment provenance and the source of trace metals in seawater proven ance changes of silicate sediment deposited during the late cenozoic 5 0 ma on the alpha ridge central arctic ocean are determined from variations in strontium lead and neodymium isotope compositions whereas strontium and lead isotope compositions are relatively invariant from 5 to 1 7 ma end values start to increase at 3 ma subsequently s7sr 86sr and 2 6pb z 4pb ratios progressively increase and end values progressively decrease from 1 7 ma to the present day from these isotope variations three different endmember compositions for sediment source regions are defined the two endmember compositions defined by sediment that was deposited by sea ice from 5 to 1 7 ma are consistent with a significant component being derived from the east siberian shelf the progressive change in isotope compositions from 1 7 to 0 ma correlates with an increase in coarse detritus deposited by icebergs over this time period the isotope data are consistent with a progressive increase in the proportion of sediment that was deposited in the central arctic ocean since 1 7 ma having been derived from the northern canada or queen elizabeth island region changes in the strontium neodymium and lead isotope compositions of arctic seawater are determined by analyzing the oxide fractions of fe mn micronodules strontium isotope compositions of the microno dule oxide fractions are similar to published late cenozoic seawater data and indicate that 97 of the sr is seawater derived but minute contributions of sr from silicate detritus prohibit using the hydrogenous fractions for chronostratigraphic purposes neodymium and lead isotope compositions of the micronodule oxide fractions which reflect those of arctic seawater follow the isotope variations of the silicate components throughout the late cenozoic these relations indicate that river water can not be the primary source of rare earth elements and pb to the dissolved reservoir in arctic seawater and that ice rafted detritus by dissolution or exchange processes is an important source of trace elements to seawater in ice covered oceans copyright 1997 elsevier science ltd

constancy of oceanic deposition of 10 be as recorded in manganese crusts recent developments in the use of nuclear accelerators as high energy mass spectrometers1 2 has renewed interest in the study of cosmogenic 10be half life 1 5 myr as a geophysical time tracer the accelerator techniques enable us to measure 10be with a sensitivity of 107 atoms or less the potential age dating span with 10be is extended back to at least 15 myr bp providing that the 10be production and deposition rates remain constant or nearly constant over that time period a knowledge of the long term constancy in the 10be deposition rate is thus very important we report here the first detailed measurements of 10be and 9be in two ferromanganese oxide crusts from the sea floor of the equatorial atlantic sample no k 9 21 and the north pacific schw 1d each representing 10 myr of accumulation history we have found in these crusts that the deposition of 10be and 10be 9be during the past 7 9 myr has been constant averaged over time intervals of 1 myr the variation is within 6 before that time both crusts show similar significant deviations the 7 9 myr demarcation may be related to the reported late miocene global abyssal circulation change in the ocean3

response of beryllium and radiogenic isotope ratios in northern atlantic deep water to the onset of northern hemisphere glaciation we present time corrected ratios of cosmogenic 10 be to continent sourced 9 be from two fe mn crusts located in the northwest atlantic in the vicinity of the origins of northern atlantic deep water nadw these initial 10 be 9 be ratios represent those of the ambient seawater at the time of crust deposition and are constant through the last 7 ma previously published isotope records of radiogenic nd and pb isotopes in these crusts show a decrease in e nd and an increase in 206 pb 204 pb over the last 5 ma these changes are consistent with a gradual increase in the strength of nadw production an increase in the labrador sea component of nadw or an increased erosional input into the labrador sea since the residence time and thus the advective length scales of dissolved be in deep water lie between those of nd and pb records of 10 be 9 be should contrary to what has been observed vary in phase with those of nd and pb if these variations are accounted for by changes in paleo circulation the observed variations in radiogenic nd and pb isotopes are therefore most likely to be explained with an increase in the flux of terrigenous matter in the area of the labrador sea which was possibly associated with ice rafting and selective leaching of pb isotopes an increase of the labrador sea component that contributes to nadw is also a possibility but based on the 10 be 9 be data presented here large scale changes in the thermohaline circulation can be discounted as cause for the radiogenic tracer variations

regional geochemical and isotopic variations of northern new england plutons implications for magma sources and for grenville and avalon basement terrane boundaries examination of 36 devonian to pennsylvanian plutons within and bordering the central maine terrane of north central new england shows that the plutons display regional variations in mineral and bulk rock compositions and in oxygen and sulfur isotopic values these variations principally correspond to the regional structural trends of the central maine terrane plutons located on that terrane9s southeast flank adjacent to the massabesic gneiss complex on the terrane9s northwest flank adjacent to the bronson hill anticlinorium and in the northeastern part of the northeast kingdom batholith of vermont have biotites with relatively low al concentrations 3 0 cations p f u per formula unit feldspars with higher an contents up to an 55 and lower 18 o 7 9 and higher 34 s average of 0 bulk rock values compared to plutons within the central parts of the central maine terrane these characteristics are most compatible with metaigneous or metavolcaniclastic sources as shown by others the plutons of vermont and northwestern maine preserve grenvillian signatures whereas those of southeastern new hampshire specifically the granite at milford new hampshire and maine sebago phillips have avalonian signatures within the interior of the central maine terrane of new hampshire and western maine and in the connecticut valley trough of vermont are distinct regions characterized by peraluminous plutons with high 18 o values 11 low 34 s values 25 al rich biotites 3 3 cations p f u and an poor plagioclases 30 these data are most consistent with metasedimentary sources or at a minimum the plutons were heavily contaminated by central maine terrane or connecticut valley trough metasedimentary rocks these peraluminous plutons do not have the unique signatures of the underlying precambrian basements several metaluminous plutons in the central maine terrane along the new hampshire maine border separate two regions of peraluminous plutons these metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the central maine terrane indicative of less paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the central maine terrane the metaluminous plutons also have trace element characteristics identical to those of plutons in vermont and northwestern maine that have a known grenville basement terrane source we suggest that the metaluminous plutons also had sources in the underlying grenville basement and that the grenville avalon boundary cuts diagonally across northern new england because the plutons with grenvillian signatures are adjacent to plutons with avalonian signatures the sebago batholith and phillips pluton we suggest that there is no medial new england basement in this region of new england some peraluminous two mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks the components of which were deposited in an anoxic basin such as the smalls falls formation of the central maine terrane but many other plutons have compositions that are inconsistent with this model we suggest that the compositions of many new hampshire plutonic suite plutons indicate greater heat input than that available from u enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust

paleoclimatic response of the closing of the isthmus of panama in a coupled ocean atmosphere model the paleoclimatic effects of the closure of the isthmus of panama 3 ma are investigated using a coupled atmosphere ocean model consistent with earlier ocean only modelling studies it is shown that prior to closure there is an absence of deep water formation in the north atlantic hence there is a reduction in oceanic heat transport this is largely compensated for by the atmosphere such that only small changes in total planetary heat transport occur the model climate of the north atlantic is significantly warmer after isthmus closure in addition the regions surrounding the pacific ocean and south atlantic are generally cooler while the indian ocean is generally warmer in the model present day climate finally possible relationships to glaciation and initiation of northern hemisphere glacial cycles are discussed

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

an oxygen isotope and geochemical study of meteoric hydrothermal systems at pilot mountain and selected other localities carolina slate belt several epigenetic mineral deposits in the ca1rolina slate belt are intimately related to meteoric hydrothermal systems of late precambrian and early paleozoic age at pilot mountain low 18 o rocks correlate well with zones of strong silicic alteration and alkali leaching accompanied by high alumina minerals sericite pyrophyllite andalusite or topaz and anomalous concentrations of cu mo sn b and au the alteration occurs within andesitic volcanic and volcaniclastic rocks and is associated with a subvolcanic dacite porphyry stock on the southeastern slope of the mountain tilting and erosion have exposed an oblique section through the original system interpreted to expose shallower rocks to the northwest a 4 km 2 central zone of slight 18 o depletion delta 18 o sub whole rock 4 3 6 1ppm occurs on the broad and resistant silicifled western flank of pilot mountain predominantly within quartz sericite schist and quartz granofels containing pods of high a1 minerals a magmatic source for much of the sulfur and metal is likely and a subordinate magmatic water component in the fluid of the central zone is possible this central zone is surrounded by a 30 km 2 peripheral zone of low 18 o sericite schists chlorite sericite schists and andesitic volcanic rocks delta 18 o sub whole rock 3 8ppm with the lowest values 1 4ppm occurring in intensely sericitized rocks on the eastern flank of pilot mountain near the apex of the dacite porphyry stock rhyolites of the uwharrie formation delta 18 o 3 8 6 3ppm are not as strongly altered as nearby andesites and may postdate the hydrothermal alteration the fluid calculated to be in equilibrium with the lowest 18 o quartz veins and country rocks at 300 degrees or 50 degrees c would have delta 18 o approximately 4 5 or 2 0 per mil whereas analyses of radiating pyrophyllite indicate equilibrium with a fluid having delta d approximately 30 per mil consistent with a slightly 18 o shifted low latitude meteoric water subsequent greenschist metamorphism caused intermineral isotopic reequilibration in several samples and may have modified preexisting alteration assemblages but it did not destroy the large delta 18 o anomaly produced by meteoric hydrothermal activity at pilot mountain reconnaissance studies of other alteration zones in the carolina slate belt have so far disclosed the involvement of meteoric hydrothermal fluids at the snow camp pyrophyllite deposit at the hoover hill and sawyer au mines and probably at the haile and brewer au mines

altering avalonia oxygen isotopes and terrane distinction in the appalachian peri gondwanan realmlaboratory for stable isotope science lsis the university of western ontario contribution 236 distinct 18o depletion is characteristic of a large majority of the 620 550 ma felsic igneous rocks of avalonia in the northern appalachian orogen neoproterozoic rocks in the boston avalon terrane have the lowest 18owr values 3 1 followed by the mira terrane in cape breton island and the caledonia terrane in new brunswick 1 2 the avalon terrane in newfoundland 2 8 and the antigonish highlands in nova scotia 5 3 in contrast this depletion of 18o is observed in very few of the paleozoic felsic igneous rocks from these avalonian terranes and also in very few of the neoproterozoic and paleozoic felsic igneous rocks from the inboard ganderian terranes the low 18o character of the neoproterozoic igneous rocks is related to regional pervasive post magmatic alteration by predominantly meteoric hydrothermal fluids 18oh2o 6 to 4 at 200 450 c the alteration likely occurred during late neoproterozoic transtensional extension of avalonia large scale fluid infiltration and circu

assessing the role of orogen parallel faulting in post orogenic exhumation low temperature thermochronology across the norumbega fault system maine as a part of a regional effort to determine the extent of low temperature thermochronological discontinuities across major orogen parallel faults in northern new england 41 apatite fission track

evolution of the monte negro acid sulfate au ag deposit pueblo viejo dominican republic important factors in grade development this study was undertaken to determine the geologic factors that were important in making pueblo viejo which contains over 600 000 kg of gold in relatively high grade ore one of the few acid sulfate deposits to be mined economically on a large scale orebodies at pueblo viejo including monte negro the focus of this study are found around a maar diatreme complex that formed in the upper part of the lower cretaceous los ranchos formation ore is hosted by spilite spilite derived conglomerate and intramaar carbonaceous sandstone and mudstone regional seawater alteration of these rocks was overprinted by two stages of advanced argillic alteration both of which are associated with precious metals stage i produced deep alunite quartz pyrite and overlying kaolinite quartz pyrite assemblages and deposited gold in association with disseminated pyrite in wall rock stage ii which overprinted stage i produced deep pyrophyllite or diaspore and an overlying recrystallized silica cap pyrite or sphalerite or enargite veins that probably formed in response to hydrofracturing of the silica cap contain gold grades of as much as 100 ppm vein density and assay data show that approximately 60 percent of the gold at monte negro is in disseminated stage i ore with the remainder in stage ii veins phase equilibria and sulfur isotope thermometry show that stage i alteration took place below 260 degrees c at ph values between 2 and 3 stage ii pyrophyllite or diaspore formation occurred above 285 degrees c from a fluid with an initial ph of 1 to 1 5 while the silica caps formed below 220 degrees c geologic constraints indicate that stage i wall rock gold at monte negro was deposited by sulfidation of fe rich wall rock whereas stage ii vein hosted gold could have formed by cooling boiling or mixing with overlying ground water use of the programs solveq and chiller to evaluate the relative efficiencies of these ore forming processes confirms that sulfidation was capable of depositing disseminated stage i gold whereas stage ii vein gold was probably deposited by boiling related to pressure release during hydrofracturing and subsequent mixing with ground water these observations indicate that development of grade and tonnage at pueblo viejo was greatly facilitated by 1 the porous permeable maar diatreme complex which hosted large shallow hydrothermal cells and probably served as a conduit for rising magmatic gases 2 the presence of iron rich wall rock which caused early deposition of background disseminated gold by sulfidation and 3 later hydrothermal activity which formed impermeable silica caps that hydrofractured to create the high grade vein overprint remobilization of stage i background gold into stage ii veins might also have been an important factor in upgrading the deposits exploration for similar deposits should focus on areas of explosive volcanism in andesitic island arc terranes

neodymium isotopic study of baffin bay water sources of ree from very old terranes water samples were collected from baffin bay and surrounding areas in order to evaluate this region as a potential source of nd from old continental material to atlantic water the isotopic data ranged from e nd 0 9 0 to 26 with most of the data around e nd 0 20 compared with values of north atlantic deep water nadw with e nd 0 13 5 the concentration of nd in baffin bay waters was as high as 6 10 12 g g compared with 2 5 10 12 g g for nadw the combination of low e nd and high nd concentration indicates that baffin bay may be a significant source of nd from very old crustal material a simple box model was used to evaluate the contribution to the nd budget of nadw and it was concluded that a substantial fraction of the nd from ancient crustal sources that is required to maintain the isotopic composition of nadw could be supplied by baffin bay outflow

nd isotope evidence for the evolution of the paleocurrents in the atlantic and tethys oceans during the past 180 ma the nd isotopic composition of atlantic and tethys seawater as deduced from marine phosphorites varied considerably during the past 180 ma the early tethys and central atlantic seawater from 180 to 160 ma ago early middle jurassic had a nd isotopic signature identical to that of the pacific end 6 suggesting that pacific seawater entered the newly forming tethys basin however with time continental runoff draining into the young basin became more important and led to a decrease in the nd isotopic composition and finally a decoupling from the pacific ocean during the late early cretaceous 120 90 ma ago atlantic and tethys seawater reached continental crust like nd isotopic composition values end 10 which could reflect high weathering rates induced by the warm and humid climate at that time in the time span between 80 and 50 ma late cretaceous early tertiary the nd isotopic composition of both tethys and atlantic seawater strongly increased towards pacific seawater values in the case of the atlantic ocean this change has been correlated with the opening of the south atlantic which enabled the more radiogenic pacific seawater to travel westwards around the southern edge of africa into the south and finally the north atlantic in the case of tethys seawater we have to assume that during this period large masses of pacific seawater entered the tethys again directly through the indian tethys seaway the renewed importance of this seaway might be related to the late cretaceous early tertiary first order global sea level highstand the most abrupt change towards lower more crust like values occurred in the early miocene 25 17 ma ago in the south and north atlantic as well as in the tethys during this period the143nd 144nd isotope ratios dropped from 0 5124 end 4 2 down to 0 5122 end 8 this change indicates a diminishing influx of pacif seawater into the atlantic and tethys oceans and can be correlated with the breakdown of the circum equatorial circulation patterns of the world oceans due to plate tectonics from 20 to 17 ma ago miocene the nd isotopic composition of the atlantic ocean increased again and reached values as high as 0 5123 end 6 6 whereas tethys seawater remained at low values this decoupling can be correlated with the incipient complete isolation of the tethys the nd isotopic composition was henceforth controlled by the continental runoff from surrounding land masses the major increase in the nd isotopic composition observed in the atlantic can be related to the opening of the drake passage and establishment of the circum antarctic current system this allowed pacific seawater to enter directly the south atlantic around antarctica a renewed drop in the nd isotopic composition of atlantic seawater during the late miocene can be related to the onset of north atlantic bottom water formation and its subsequent upwelling

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate