Chemical and isotopic compositions of rocks, ores, and hydrothermal minerals from the Rainbow and Logachev-2 hydrothermal fields

The aim of this paper is to analyze and compare mineralogy and geochemistry of copper-zinc sulfide ores from the Logachev-2 and Rainbow hydrothermal fields of the Mid-Atlantic Ridge (MAR) confined to serpentinite protrusions. It was found that Zn(Fe) and Cu, Fe(Zn) sulfides had been deposited in black smokers pipes almost simultaneously from intermittently flowing, nonequilibrium H2S-low solutions of different temperatures. Pb isotope composition confirmed that the deep oceanic crust had been a source of lead. The ores from the Rainbow field are 20-fold higher in Co than ores restricted to basalts and show a high ratio of Co/Ni=46. The ores from the Rainbow field are enriched in 34S isotope (aver. d34S=10 per mil) because of constant flow of cold sea water into the subsurface zone of the hydrothermal system. Ores from the Logachev-2 field are 8 times higher in gold compared to other MAR regions. Sulfide ores from the Rainbow and Logachev-2 fields have no analogues among MAR ore occurrences in terms of enrichment in valuable components (Zn, Cd, Co, and Au).

Elements and isotopic measurements of magmatic rock samples from ODP Hole 104-642E

Trace element and isotopic signatures of magmatic rock samples from ODP Hole 642E at the Vøring Plateau provide insight into the interaction processes of mantle melt with crust during the initial magma extrusion phases at the onset of the continental breakup. The intermediate (basaltic-andesitic) to felsic (dacitic and rhyolitic) Lower Series magmas at ODP Hole 642E appear to be produced by large amounts of melting of upper crustal material. This study not only makes use of the traditional geochemical tools to investigate crust-mantle interaction, but also explores the value of Cs geochemistry as an additional tool. The element Cs forms the largest lithophile cation, and shows the largest contrast in concentration between (depleted) mantle and continental crust. As such it is a very sensitive indicator of involvement of crustal material. The Cs data reinforce the conclusion drawn from isotopic signatures that the felsic magmas are largely anatectic crustal melts. The down-hole geochemical variation within ODP Hole 642E defines a decreasing continental crustal influence from the Lower Series into the Upper Series. This is essential information to distinguish intrinsic geochemical properties of the mantle melts from signatures imposed by crustal contamination. A comparison with data from the SE Greenland margin highlights the compositional asymmetry of the crust-mantle interactions at both sides of the paleo-Iapetus suture. While Lower Series and Middle Series rocks from the SE Greenland margin have isotopic signatures reflecting interactions with lower and middle crust, such signatures have not been observed at the mid-Norwegian margin. The geochemical data either point to a dissimilar Caledonian crustal composition and/or to different geodynamic pre-breakup rifting history at the two NE Atlantic margin segments.

Trace metals in sediments of the Var Canyon turbidite system at the DYFAMED station in the Ligurian Sea

The magnitude and the chronology of anthropogenic impregnation by Hg and other trace metals of environmental concern (V, Cr, Ni, Cu, Zn, Ag, Cd and Pb, including its stable isotopes) in the sediments are determined at the DYFAMED station, a site in the Ligurian Sea (Northwestern Mediterranean) chosen for its supposed open-sea characteristics. The DYFAMED site (VD) is located on the right levee of the Var Canyon turbidite system, at the end of the Middle Valley. In order to trace the influence of the gravity current coming from the canyon on trace metal distribution in the sediment, we studied an additional sediment core (VA) from a terrace of the Var Canyon, and material collected in sediment traps at the both sites at 20 m above sea bottom. The patterns of Hg and other trace element distribution profiles are interpreted using stable Pb isotope ratios as proxies for its sources, taking into account the sedimentary context (turbidites, redox conditions, and sedimentation rates). Major element distributions, coupled with the stratigraphic examination of the sediment cores point out the high heterogeneity of the deposits at VA, and major turbiditic events at both sites. At the DYFAMED site, we observed direct anthropogenic influence in the upper sediment layer (<2 cm), while on the Var Canyon site (VA), the anthropization concerns the whole sedimentary column sampled (19 cm). Turbiditic events superimpose their specific signature on trace metal distributions. According to the 210Pbxs-derived sedimentation rate at the DYFAMED site (0.4 mm yr-1), the Hg-enriched layer of the top core corresponds to the sediment accumulation of the last 50 years, which is the period of the highest increase in Hg deposition on a global scale. With the hypothesis of the absence of significant post-depositional redistribution of Hg, the Hg/C-org ratio changes between the surface and below are used to estimate the anthropogenic contribution to the Hg flux accumulated in the sediment. The Hg enrichment, from pre-industrial to the present time is calculated to be around 60%, consistent with estimations of global Hg models. However, based on the chemical composition of the trapped material collected in sediment traps, we calculated that epibenthic mobilization of Hg would reach 73%. Conversely, the Cd/C-org ratio decreases in the upper 5 cm, which may reflect the recent decrease of atmospheric Cd inputs or losses due to diagenetic processes.

Profiles of diatoms, pollen, XRD, XRF and organic composition of two sediment cores (PG2022 and PG2023) from Lake Kyutyunda in Yakutia, NE Siberia, Russia

Although the climate development over the Holocene in the Northern Hemisphere is well known, palaeolimnological climate reconstructions reveal spatiotemporal variability in northern Eurasia. Here we present a multi-proxy study from north-eastern Siberia combining sediment geochemistry, and diatom and pollen data from lake-sediment cores covering the last 38,000 cal. years. Our results show major changes in pyrite content and fragilarioid diatom species distributions, indicating prolonged seasonal lake-ice cover between ~13,500 and ~8,900 cal. years BP and possibly during the 8,200 cal. years BP cold event. A pollen-based climate reconstruction generated a mean July temperature of 17.8°C during the Holocene Thermal Maximum (HTM) between ~8,900 and ~4,500 cal. years BP. Naviculoid diatoms appear in the late Holocene indicating a shortening of the seasonal ice cover that continues today. Our results reveal a strong correlation between the applied terrestrial and aquatic indicators and natural seasonal climate dynamics in the Holocene. Planktonic diatoms show a strong response to changes in the lake ecosystem due to recent climate warming in the Anthropocene. We assess other palaeolimnological studies to infer the spatiotemporal pattern of the HTM and affirm that the timing of its onset, a difference of up to 3,000 years from north to south, can be well explained by climatic teleconnections. The westerlies brought cold air to this part of Siberia until the Laurentide ice-sheet vanished 7,000 years ago. The apparent delayed ending of the HTM in the central Siberian record can be ascribed to the exceedance of ecological thresholds trailing behind increases in winter temperatures and decreases in contrast in insolation between seasons during the mid to late Holocene as well as lacking differentiation between summer and winter trends in paleolimnological reconstructions.

Element concentrations and isotopic composition of tholeiite and whole rocks samples from the east Mariana Basin

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40Ar/39Ar ages determined in this study for two Leg 129 basalts average 114.6 +/- 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites (87Sr/86Sr init = 0.70360-0.70374; 143Nd/144Nd init = 0.512769-0.512790; 206Pb/204Pb meas = 18.355-18.386) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 +/- 1.0 Ma 40Ar/39Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Geochemistry and Nd-Pb isotope ratios of sediments from the Lesser Antilles arc

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. (1985, doi:10.1016/0016-7037(85)90082-1) and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta8/4 values that characterize the Lesser Antilles arc.

Nd and Pb isotope signatures of the clay-size fraction of sediment core MD99-2227

Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan-African and Variscan crusts, and the Mid-Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2-6.5 ka) is composed of material derived from the North American shield and from a "young" crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid-Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

Gotland Deep (Baltic Sea) independent age model based on Palaeomagnetic Secular Variation (PSV) and lead (Pb) deposition profiles

Dating of sediment cores from the Baltic Sea has proven to be difficult due to uncertainties surrounding the 14C reservoir age and a scarcity of macrofossils suitable for dating. Here we present the results of multiple dating methods carried out on cores in the Gotland Deep area of the Baltic Sea. Particular emphasis is placed on the Littorina stage (8 ka ago to the present) of the Baltic Sea and possible changes in the 14C reservoir age of our dated samples. Three geochronological methods are used. Firstly, palaeomagnetic secular variations (PSV) are reconstructed, whereby ages are transferred to PSV features through comparison with varved lake sediment based PSV records. Secondly, lead (Pb) content and stable isotope analysis are used to identify past peaks in anthropogenic atmospheric Pb pollution. Lastly, 14C determinations were carried out on benthic foraminifera (Elphidium spec.) samples from the brackish Littorina stage of the Baltic Sea. Determinations carried out on smaller samples (as low as 4 µg C) employed an experimental, state-of-the-art method involving the direct measurement of CO2 from samples by a gas ion source without the need for a graphitisation step - the first time this method has been performed on foraminifera in an applied study. The PSV chronology, based on the uppermost Littorina stage sediments, produced ten age constraints between 6.29 and 1.29 cal ka BP, and the Pb depositional analysis produced two age constraints associated with the Medieval pollution peak. Analysis of PSV data shows that adequate directional data can be derived from both the present Littorina saline phase muds and Baltic Ice Lake stage varved glacial sediments. Ferrimagnetic iron sulphides, most likely authigenic greigite (Fe3S4), present in the intermediate Ancylus Lake freshwater stage sediments acquire a gyroremanent magnetisation during static alternating field (AF) demagnetisation, preventing the identification of a primary natural remanent magnetisation for these sediments. An inferred marine reservoir age offset (deltaR) is calculated by comparing the foraminifera 14C determinations to a PSV & Pb age model. This deltaR is found to trend towards younger values upwards in the core, possibly due to a gradual change in hydrographic conditions brought about by a reduction in marine water exchange from the open sea due to continued isostatic rebound.

Element contents and isotope composition of sediments and basalts from DSDP Site 78-543

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb (206Pb/204Pb up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and 143Nd/144Nd increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas. Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of "normal" depleted MORB.

Sr, Nd and Pb isotopic composition of volcanic and volcaniclastic samples of ODP Leg 134

A suite of volcanic and volcaniclastic rocks selected from Ocean Drilling Program Leg 134 Sites 832 and 833 in the North Aoba Basin (Central New Hebrides Island Arc) has been analyzed for Sr, Nd, and Pb isotopes to investigate the temporal evolution of the arc magmatism. This arc shows two unusual features with respect to other western Pacific arcs: 1) subduction is eastdirected; and 2) a major submarine ridge, the d'Entrecasteaux Zone, has been colliding almost perpendicularly with the central part of the arc since about 3 Ma. Volcanic rocks from the upper parts of both holes, generated during the last 2 m.y., show higher 87Sr/86Sr and significantly lower 206Pb/204Pb and 143Nd/144 Nd values compared to those volcanics erupted before the collision of this ridge, as represented by samples from the lower section of both holes, or remote from the collisional region, in the southern part of the arc. These isotopic differences in the respective mantle sources cannot be interpreted in terms of geochemical input into the mantle wedge induced by the collision itself. Rather, they require long term (>500 m.y.) enrichment processes. The enriched mantle source could be, on a regional scale, a DUPAL-type reservoir with strong similarities to the source of Indian Ocean basalts. Isotopic analyses of drilled rocks from the DEZ show that the anomalous, enriched mantle component is not derived from this feature. We currently cannot identify a source for this enriched component, but note that it also exists in Lau Basin backarc volcanics, lavas from the West Philippine Sea, and also some lavas from the Mariana-Izu-Bonin arc.

Chemical composition of Ninetyeast Ridge basalts

On Leg 121 of the Ocean Drilling Program, we recovered basaltic rocks from a total of three basement sites in the southern, central, and northern regions of Ninetyeast Ridge. These new sites complement the previous four basement holes drilled during Legs 22 and 26 of the Deep Sea Drilling Project, and confirm the predominantly tholeiitic, light rare earth element-enriched character of the basalts that cap the ridge. The basalts show marked iron enrichment; ferrobasalts occur at Sites 214 and 216 and oceanic andesites at Site 253. All of the basalts recovered during Leg 121 are altered, and range from aphyric olivine tholeiites (Site 756), to strongly plagioclase-phyric basalts (Site 757). Basalts from Site 758, which were clearly erupted in a submarine environment (pillow basalts are present in the section), are sparsely to strongly plagioclase-phyric. The basalts recovered at any one hole are isotopically homogeneous (except for the basalts from Site 758, which show a range of Pb isotopes), and it is possible to relate the magmas at any one site by high-level fractionation processes. However, there are significant variations in isotope ratios and highly incompatible element ratios between sites, which suggest that the mantle source for the ridge basalts was compositionally variable. Such variation, in view of the large volume of magmatic products that form the ridge system, is not surprising. There is not, however, a systematic variation in basalt composition along the ridge. We agree with previous models that relate Ninetyeast Ridge to a mantle plume in the southern Indian Ocean. The tholeiitic, iron-enriched, and voluminous character of the ridge basalts is typical of oceanic islands associated with plumes on or near a mid-ocean ridge (e.g., Iceland, Galapagos Islands, and St. Paul/Amsterdam islands). The absence of recovered alkalic suites is inconsistent with an intraplate setting, such as the Hawaiian Islands or Kerguelen Island. Thus, the major element data, like the gravity data, strongly suggest that the ridge was erupted on or very close to an active spreading center. Isotopically, the most likely plume that created the excess magmatism on the Ridge is the Kerguelen-Heard plume system, but the Ninetyeast Ridge basalts do not represent a simple mixing of the Kerguelen plume and mid-ocean Ninetyeast Ridge basalt mantle.

Mineralogical and isotopic compositions of ODP Hole 107-650A

Red-brown dolomitic claystones overlay the Marsili Basin basaltic basement at ODP Site 650. Sequential leaching experiments reveal that most of the elements considered to have a hydrothermal or hydrogenous origin in a marine environment, such as Fe, Cu, Zn, Pb, Co, Ni, are present mainly in the aluminosilicate fraction of the dolomitic claystones. Their vertical distribution, content and partitioning chemistry of trace elements, and REE patterns suggest enhanced terrigenous input during dolomite formation, but no significant hydrothermal influence from the underlying basaltic basement. Positive correlations in the C and O isotopes in the dolomites reflect complex conditions during the dolomitization. The stable isotopes can be controlled in part by temperature variations during the dolomitization. Majority of the samples, however, form a trend that is steeper than expected for only temperature control on the C and O isotopes. The latter indicates possible isotopic heterogeneity in the proto-carbonate that can be related to arid climatic conditions during the formation of the basal dolomitic claystones. In addition, the dolostones stable isotopic characteristics can be influenced by diagenetic release of heavier delta18O from clay dehydration and/or alteration of siliciclastic material. Strontium and Pb isotopic data reveal that the non-carbonate fraction, the "dye" of the dolomitic claystones, is controlled by Saharan dust (75%-80%) and by material with isotopic characteristics similar to the Aeolian Arc volcanoes (20%-25%). The non-carbonate fraction of the calcareous ooze overlying the dolomitic claystones has a Sr and Pb isotopic composition identical to that of the dolomitic claystones, indicating that no change in the input sources to the sedimentary basin occurred during and after the dolomitization event. Combination of climato-tectonic factors most probably resulted in suitable conditions for dolomitization in the Marsili and the nearby Vavilov Basins. The basal dolomitic claystone sequence was formed at the initiation of the opening of the Marsili Basin (~2 Ma), which coincided with the consecutive glacial stage. The glaciation caused arid climate and enhanced evaporation that possibly contributed to the stable isotope variations in the proto-carbonate. The conductive cooling of the young lithosphere produced high heat flow in the region, causing low-temperature passive convection of pore waters in the basal calcareous sediment. We suggest that this pumping process was the major dolomitization mechanism since it is capable of driving large volumes of seawater (the source of Mg2+) through the sediment. The red-brown hue of the dolomitic claystones is terrigenous contribution of the glacially induced high eolian influx and was not hydrothermally derived from the underlying basaltic basement. The detailed geochemical investigation of the basal dolomitic sequence indicates that the dolomitization was most probably related to complex tectono-climatic conditions set by the initial opening stages of the Marsili Basin and glaciation.