Stephen J. G. Galer

The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot

We present comprehensive radiogenic isotope (Os, Pb, Hf, Nd, Sr) and trace element data on basaltic lavas from Pitcairn Island and the Pitcairn seamounts and examine the origin of the enriched mantle isotopic signature (EM-1) found in these lavas. The 187Os/188Os ratios of the lavas range from 0.131 to 0.254, while those of the high-Os concentration samples (>50 pg/g) lie between 0.131 and 0.148. All 187Os/188Os ratios are higher than the bulk silicate Earth reference value of 0.127. Since ancient subcontinental lithospheric mantle (SCLM) is expected to have a 187Os/188Os ratio less than 0.127, it appears that recycled SCLM plays no role in the Pitcairn source. Variations in 187Os/188Os ratios appear to be unconnected with those of 206Pb/204Pb ratios in Pitcairn lavas, suggesting that Pb and Os isotopic variations are controlled by different factors. Modeling shows that variations in Pb isotopic compositions may mainly reflect the proportion of recycled sediment in the source, while those of 187Os/188Os ratios may reflect the proportion of peridotite mantle versus recycled oceanic crust. The occurrence of negative Nb anomalies in some of the lavas, a correlation between Nb anomaly and 87Sr/86Sr ratios (0.7036–0.7051), and extremely unradiogenic and strongly correlated Nd and Hf isotopic compositions (ϵNd of −5.9 to +1.1 and ϵHf of −5.3 to +2.2) together suggest that the Pitcairn mantle source contains a recycled continental crustal component. The slope of the ϵHf vs. ϵNd correlation is shallower for Pitcairn Island than for the Pitcairn seamounts or the global OIB array, and may be due to a variable ratio of recycled mud to sand in the Pitcairn source. A trace element mixing model also indicates the presence of small amounts of recycled pelagic and terrigenous sediment and permits variable amounts of depleted components such as recycled MORB, gabbro and depleted mantle. The 206Pb/204Pb ratios of the Pitcairn lavas vary between 17.47 and 18.10 and are very unradiogenic compared to those of other ocean island basalts. By contrast, 208Pb/204Pb ratios are high and relatively homogeneous at values of ∼39.0. This observation along with the measured Th/U ratios of the lavas, which range up to 14.1, indicate a long-term history of U loss in the Pitcairn source. In 207Pb/204Pb–206Pb/204Pb space, the data form a linear array that can be interpreted in terms of mixing between a minor recycled sediment end member and more depleted material. Lead isotopic compositions suitable for the recycled end member were investigated using a three-stage evolution model by Monte Carlo methods and suggest ages between 0.7 and 1.9 Ga for the recycled sediment. The relationships between measured Th/U and radiogenic 208Pb*/206Pb* ratios suggest that the isotopic arrays displayed by the lavas were produced by mixing, probably occurring during magma genesis.

Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume

The two parallel chains of Hawaiian volcanoes (‘Loa’ and ‘Kea’) are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples a more peripheral portion of the plume. Using high-precision lead isotope data for both centrally and peripherally located volcanoes, we show here that the two trends have very little compositional overlap and instead reveal bilateral, non-concentric plume zones, probably derived from the plume source in the mantle. On a smaller scale, along the Kea chain, there are isotopic differences between the youngest lavas from the Mauna Kea and Kilauea volcanoes, but the 550-thousand-year-old Mauna Kea lavas are isotopically identical to Kilauea lavas, consistent with Mauna Keas position relative to the plume, which was then similar to that of present-day Kilauea. We therefore conclude that narrow (less than 50 kilometres wide) compositional streaks, as well as the larger-scale bilateral zonation, are vertically continuous over tens to hundreds of kilometres within the plume.

High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project

Abstract We report Pb isotopic compositions for 35 samples of the volcanoes Mauna Loa and Mauna Kea from the Hawaiian Scientific Drilling Project (HSDP-1) core at Hilo. These data were obtained with an external precision of ∼100 ppm (2σext.) on the ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb by using a Pb triple spike to correct for instrumental mass fractionation. The Pb isotopic compositions in the lower section (1200 to 280 m) of the core sample 200 to 400 ka-old Mauna Kea lavas, and display two well-defined linear arrays in 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb isotope spaces. There is a suggestion that Mauna Loa (0 to 280 m depth) also displays such linear array(s). However, analysis of the Mauna Loa samples is complicated by residual contamination and/or sample heterogeneity. While these latter data exhibit a satisfactory array in 208Pb/204Pb vs. 206Pb/204Pb, there still remains scatter in 207Pb/204Pb–206Pb/204Pb space, making it difficult to assess the true Pb isotope systematics of Mauna Loa. The presence of two linear Pb isotopic arrays in Mauna Kea can be interpreted as either reflecting two parallel isochrons or in terms of binary mixing. If interpreted as isochrons, the 207Pb/204Pb–206Pb/204 Pb systematics correspond to an age of ∼1.9 Ga. Comparison of measured Th/U ratios in the lavas and those inferred from Pb isotope systematics strongly suggest that the Pb isotopic arrays reflect binary mixing, and this bears directly on the question of how many distinct components are present in the Hawaiian plume. Most of the new Mauna Kea data lie well outside the mixing-component triangle defined in the literature by the “Kea”, “Loihi”, and “Koolau” components. On the basis of the relationships between Pb isotope ratios in 3D and a principal component analysis of the Mauna Kea Pb isotope dataset, we show here that a three-component mixing model can in principle explain both mixing lines. However, such an explanation requires a highly specific set of mixing conditions in order to produce parallel arrays in Pb isotope space (2D and 3D). Therefore, our preferred interpretation is that the two arrays reflect binary mixing, with four discrete source components involved in the generation of the Kea lavas. Comparison of the Pb isotope characteristics of these lavas with those of East Pacific Rise (EPR) MORB glasses further suggests that EPR-type Pacific lithosphere does not contribute to the source of Kea lavas. The position of samples along the mixing lines does not correlate with stratigraphic height in the core, and therefore the age of the lavas. Rather, it appears as though the relative proportions of the endmembers are controlled by the spatial configuration of these endmembers, and by melting and transport processes in the source itself. The stratigraphic fluctuations of Pb and Sr isotopes contrast with the monotonic decrease of eNd and eHf values as a function of age. This may in part be explained by differences in analytical precision of isotope measurements relative to the total range of values observed. This analytical resolution is far higher for Pb than for the other radiogenic isotopes. Alternatively, the observed fluctuation may be caused by the mobility of lead (as well as Rb and/or Sr) during the ancient differentiation process that created the differences in parent–daughter ratios.

Pb and Nd isotopes in NE Atlantic Fe-Mn crusts : proxies for trace metal paleosources and paleocean circulation

Abstract We report high precision Pb isotopic data (2σ ≤ 100 ppm) together with Nd isotopes on depth profiles from two Fe–Mn crusts from the eastern Atlantic basin. The profiles provide a 13 Ma record of changes in eastern North Atlantic Deep Water (ENADW), and over the past 8 Ma for Mediterranean Outflow Water (MOW). Pb isotope ratios in the two records display gradual and systematic changes, tracking each other through time. The highly precise Pb isotopic data also resolve, for the first time, changes in all three Pb isotope ratios in a single Fe–Mn crust, and exhibit well-defined binary mixing lines in Pb isotope space. The Pb isotopic record of the Tropic Seamount crust shows that eastern Atlantic Pb has been dominated by binary mixing throughout the last 13 Ma. The Pb binary mixing lines further demonstrate that a change in Pb provenance to the eastern Atlantic occurred at 8 and 4 Ma. This is shown by the distinctive Pb isotopic compositions of the mixing components in the time intervals 4–0 Ma and 13–8 Ma. Consideration of Pb and Nd isotope systematics show that from 4 Ma to the present, the two endmember components correspond to NADW and Southern Component Water (SCW). The quasi-cyclic character of 206Pb/204Pb variations, as well as age progressive changes in the Pb mixing proportions of NADW and SCW endmembers, appear to reflect changes in deep water circulation. In particular, strengthening of the Pb and Nd isotopic signal associated with NADW since 3–4 Ma in both the eastern and western Atlantic basins implies that NADW advection from the western into the eastern Atlantic has been in operation over the past 4 Ma. During the period from 13 to 8 Ma, two entirely different sources of Pb and Nd existed in the eastern Atlantic. The radiogenic Pb and unradiogenic Nd component may have originated in the Norwegian–Greenland seas and the low 206Pb/204Pb—high eNd endmember component in the Southern ocean or Tethys. This suggestion is supported by evidence from paleoceanographic studies indicating the importance of Greenland–Scotland ridge activity in Miocene deep water production, and also the presence of a distinctive high δ13C water mass in the Southern Ocean, derived either locally or flowing in via Tethys. The 8 Ma record of the Lion Seamount crust (65GTV) demonstrates that Pb and Nd sources in MOW switched from predominantly internal, Mediterranean (European) sources prior to 4 Ma, to mainly external (Saharan) sources after 4 Ma. The gradual increase of Pb isotope ratios seen following the end of the Messinian reflects enhanced input of Saharan dust into the water column in the eastern Atlantic. The strengthening of the “Saharan” isotopic signal from about 4 Ma also matches the documented increase in aridity and dustiness in the Saharan and sub-Saharan regions and coincides with the re-establishment of water exchange between the Atlantic and Mediterranean after the Messinian. Although Pb and, to a lesser extent, Nd isotope ratios are distinct during the Messinian (6.5–5 Ma), there is no clear evidence for either a shutdown of MOW or a stronger North Atlantic signal during this period. The fact that a similar isotopic signal is observed, with an even higher amplitude in the ENADW record, shows that this signal is a feature of the whole eastern Atlantic. From 3 to 4 Ma ago, a source of radiogenic Pb and unradiogenic Nd appears to have dominated not only the eastern Atlantic but the world oceans, since it is seen ubiquitously in other Fe–Mn crusts at this time. This isotopic signal must be conveyed around the globe via the ocean circulation or the atmosphere. While the closure of the Panama gateway may have played an important role in these global changes in source(s) and/or fluxes of Pb and Nd to the oceans 3–4 Ma ago other events, in conjunction, such as the emergence of the Greenland–Scotland ridge, were probably also as influential.

The 7–13 March 2006 major Saharan outbreak: Multiproxy characterization of mineral dust deposited on the West African margin

Mineral dust deposits were collected at Mbour, Senegal, throughout the spring of 2006 and especially during the well-documented March 7-13 large Saharan dust outbreak. During this 7-day period, significant changes in mass flux, grain-size, clay mineralogy and Sr and Nd isotopic compositions were recorded, indicating distinct provenances for the dust transported and deposited during and outside the event. All these terrigenous proxies, as well as freshwater diatom taxa, also showed significant temporal variations during the outbreak, implying contributions from at least two different provenance regions. Tri-dimensional back-trajectories and satellite imaging enabled us to link those distinct signatures to regions increasingly to the southeast within a large area covering Mauritania, Mali and southern Algeria, identified by the Total Ozone Mapping Spectrometer (TOMS) as the main source of the prominent winter/spring plume over the tropical Atlantic. The multiproxy characterization of the March 7-13 dust fall therefore enables us to typify the terrigenous signature of two different regions supplying dust off West Africa, and provide valuable clues for the interpretation of Northeastern Tropical Atlantic Ocean dust sedimentary records in terms of changes in provenance regions and transport systems. Additionally, because dust deposition data are scarce, flux and grain size data obtained in this study, among other parameters such as clay assemblages, provide important constraints for atmospheric transport models and dust deposition budget estimates in this area.

Geographic control on Pb isotope distribution and sources in Indian Ocean Fe-Mn deposits

Abstract High-precision Pb isotope data have been obtained on Indian Ocean Fe-Mn deposits with a large geographic coverage. These data reveal a strong geographic control on the distribution of Pb isotopes as well as sources of Pb within this basin. The provinciality of Pb isotopes at the scale of the whole Indian Ocean, as well as that of individual basins, broadly matches the pattern of deepwater flow. The existence of several sources of Pb is best illustrated by the presence of three well-defined Pb isotopic arrays, each of which is confined to clear-cut geographic domains. These arrays imply a dominance of binary mixing of Pb sources within each of these domains. The domains consist of the North-Indian (N-Indian; north of 20°S), Southwest-Indian (SW-Indian; 20°S to 50°S, west of 45°E), South-Indian (S-Indian; 20°S to 50°S, east of 45°E) and Antarctic-Indian (A-Indian; south of 50°S), and clearly exhibit a strong control by latitude. The S-Indian domain and Mozambique Channel samples form a cluster, suggesting that there are more than just two sources contributing in these regions. We show that the N-Indian is dominated by sources of Pb derived from the High Himalayas and the Trans-Himalayan Complex, and most likely inherited from interaction at the water interface with Bengal and Indus Fan sediments. The SW-Indian and A-Indian domains share a common unradiogenic component, represented by circumpolar waters derived from the Pacific Ocean and flowing through the Drake Passage into the Atlantic and Indian Oceans. However, the radiogenic sources of Pb in these two domains are clearly distinct. In the SW-Indian, the radiogenic Pb signal reflects the tongue of North Atlantic Deep Water flowing around the tip of South Africa, while in the A-Indian, the source of radiogenic Pb is less certain, but quite probably originates in the Weddell Sea region. These data illustrate the complexity of the spatial Pb isotopic distribution, and Pb sources, within a single ocean basin at the present day, which should be borne in mind when interpreting long-term radiogenic isotope paleorecords from Fe-Mn crusts.

Separation of Ce from other rare-earth elements with application to SmNd and LaCe chronometry

The rare-earth element (REE) pairs SmNd and LaCe include three radioactive decay systems that are used in geo- and cosmochemistry for dating and tracer studies. Efficient chemical separation techniques are required prior to isotopic analysis of Ce and Nd. Matrix elements with low work functions must be removed from samples, since they can have a severe effect on ionization efficiency; isobars from other elements must also be eliminated in order to reduce interference corrections. These steps are particularly important for high-precision measurements, such as for 142Nd/144Nd or Ce isotope analyses. Here we report a simple but highly efficient two-phase micro-extraction technique for the separation of Ce from the other REE. The procedure involves oxidation of Ce3+ to Ce4+ using a solution of NaBrO3 in nitric acid and extraction of the tetravalent species into an organic solvent containing HDEHP. The other lanthanides remain trivalent and are thus left in the aqueous nitric acid phase. The method is characterized by high elemental yields, excellent separation factors and low reagent blank contributions. The solvent extraction method has been successfully used for (1) cleaning of Nd from Ce contamination and (2) the isolation of Ce from other REE for mass spectrometry. Isotopic analyses of the extracted samples showed minimal isobaric interferences, demonstrating that the technique achieves separation factors superior to standard procedures.

High-precision cadmium stable isotope measurements by double spike thermal ionisation mass spectrometry

Natural mass-dependent fractionation (MDF) of cadmium isotopes is a promising new tool for investigating Cd pathways and cycling in geological and biological materials. One interesting new application is as chemical tracer of deep water circulation and nutrient distribution in the oceans. But since natural isotope fractionation of Cd appears to be extremely limited and Cd abundances low, excellent external precision and sensitivity are needed to make full use of its potential. Here, we describe a newly-developed double spike (DS) method for determining the MDF of Cd isotopes. For inorganic matrices, samples are spiked with a mixed 106Cd-108Cd tracer prior to high-purity anion exchange-based separation of Cd. Isotope measurements are performed by TIMS using a silica gel activator. Overall, the DS-TIMS technique offers benefits in terms of superior precision and sensitivity compared to MC-ICPMS methods currently in use. External precisions on 100-ng-sized Cd standard loads, double spiked as unknowns, are ±14 ppm on the 112Cd/110Cd ratio (2SD), while ionization efficiencies (ions per atom loaded) are ∼0.3%. Using this technique, we calibrated the relative difference in MDF between several Cd standard materials used in various laboratories as “zero reference standards”. We also show that the 112Cd/110Cd ratio can be fractionated by as much as ∼0.2% by ion-exchange elution, which is potentially problematic for Cd isotope studies not using a double spike.

A Chinese Imprint in Insoluble Pollutants Recently Deposited in Central Greenland As Indicated by Lead Isotopes

A unique ∼ 10 year record of the lead isotopic composition of airborne insoluble particulate matter deposited in central Greenland was extracted from recent snow layers at NorthGRIP (75.1°N, 042.3°W; elevation 2,959 m), spanning the years 1989-2001. Comparison with lead isotopic signatures of both natural and anthropogenic northern hemisphere (NH) aerosol sources shows that human activities must have accounted for most of the insoluble lead deposited on Greenland during the late 1990 s, exceeding by far the natural contribution from large Asian mineral dust inputs. Lead isotopes imply predominance with time of European/Canadian sources over U.S.-derived lead, with an admixed signature typical of Chinese anthropogenic lead sources. The relative contribution of the latter shows a marked seasonal increase during spring. Our record also suggests that Chinas weight in the overall supply of insoluble pollutants deposited on Greenland was growing over the past decade of the 20th century.

Mobility in Thuringia or mobile Thuringians: A strontium isotope study from early medieval Central Germany

Residential changes of people during the Migration Period are crucial for archaeological research. Within an extensive study of the migration of the Langobards, strontium isotope analysis was carried out on tooth enamel taken from 48 burials from the Thuringian cemeteries of Rathewitz and Obermöllern (Burgenlandkreis, Sachsen-Anhalt, Germany), which date to the late 5th–mid 6th century. Modern vegetation and water samples provided detailed information about the isotopic composition of the biologically available strontium of geological units in the area. Although the rich furnishing of the burials provides evidence for contacts with many different regions, only one individual (7.1%) in Rathewitz and three (12.5%) in Obermöllern are isotopically nonlocal to the sites. These individuals were buried among the locals and their graves were similarly equipped. In contrast, many nonlocal grave goods were found with isotopically local individuals, often in combination with local items or pieces indicating several different source areas. This suggests the existence of strong interregional ties among the members of the local elites. The cemeteries cannot overall be associated with newly arriving groups; rather, they resulted from a change of funeral customs of the indigenous population from cremation to inhumations or small-scale changes of the burial places. They reflect individual residential changes rather than large-scale movements of groups.