Alan Zindler

Element transport from slab to volcanic front at the Mariana arc

We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near-constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid-oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U-230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.

Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity

Basalts from young seamounts situated within 6.8 m.y. of the East Pacific Rise, between 9° and 14°N latitude, display significant variations in 143Nd/144Nd (0.51295–0.51321), 87Sr/86Sr (0.7025–0.7031), and(La/Sm)N (0.415–3.270). Nd and Sr isotope ratios are anti-correlated and form a trend roughly parallel to the “mantle array” on a143Nd/144Nd vs.87Sr/86Sr variation diagram. Nd and Sr isotope ratios display negative and positive correlations, respectively, with(La/Sm)N. The geochemical variations observed at the seamounts are nearly as great or greater than those observed over several hundred kilometers of the Reykjanes Ridge, or at the islands of Iceland or Hawaii. Samples from one particular seamount, Seamount 6, display nearly the entire observed range of chemical variations, offering an ideal opportunity to constrain the nature of heterogeneities in the source mantle. Systematics indicative of magma mixing are recognized when major elements, trace elements, trace element ratios, and isotope ratios are compared with each other in all possible permutations. The source materials required to produce the end-member magmas are: (1) a typical MORB-source-depleted peridotite; and (2) a relatively enriched material which may represent ancient mantle segregations of basaltic melt, incompletely mixed remnants of subducted ocean crust, or metasomatized peridotite such as that found at St. Pauls Rocks or Zabargad Island. Due to the proximity of the seamounts to the East Pacific Rise (EPR), the source materials are thought to comprise an intimate mixture in the mantle immediately underlying the seamounts and the adjacent EPR. Lavas erupted at the ridge axis display a small range of isotopic and incompatible trace element compositions because the large degrees of melting and presence of magma chambers tend to average the chemical characteristics of large volumes of mantle. If the postulated mantle materials, with large magnitude, small-scale heterogeneities, are ubiquitous in the upper mantle, chemical variations in basalts ranging from MOR tholeiites to island alkali basalts may reflect sampling differences rather than changes in bulk mantle chemistry.

The isotope systematics of a juvenile intraplate volcano" Pb, Nd, and Sr isotope ratios of basalts from Loihi Seamount, Hawaii

Sr, Nd, and Pb isotope ratios for a representative suite of 15 basanites, alkali basalts, transitional basalts and tholeiites from Loihi Seamount, Hawaii, display unusually large variations for a single volcano, but lie within known ranges for Hawaiian basalts. Nd isotope ratios in alkali basalts show the largest relative variation (0.51291–0.51305), and include the nearly constant tholeiite value ( ∼ 0.51297). Pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue Tatsumotos [31] “Loa” trend towards higher 206Pb/204Pb ratios, resulting in a substantial overlap with the “Kea” trend. 206Pb/204Pb ratios for Loihi and other volcanoes along the Loa and Kea trends [31] are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of Hawaiian tholeiites. Loihi lavas display no correlation of Nd, Sr, or Pb isotope ratios with major element compositions or eruptive age, in contrast with observations of some other Hawaiian volcanoes [38]. Isotope data for Loihi, as well as average values for Hawaiian volcanoes, are not adequately explained by previously proposed two-end-member models; new models for the origin and the development of Hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere.

Neodymium isotopes as tracers in marine sediments and aerosols: North Atlantic

Nd and Sm concentrations and Nd isotopic compositions were measured on the detrital fraction of deep-sea surface sediments and on aerosols sampled over the North Atlantic and Mediterranean Sea. The data, expressed aseNd(0) and as model ages, ranged from−14.5 < eNd(0) < +4.9 and from 0.01 to 2.21 Ga, respectively. Two areas where the sediments contain relatively less radiogenic Nd are to the west off North Africa and eastward of the Canadian shield. This distribution is consistent with other tracers in suggesting that a significant fraction of the sediments are derived from aeolian material transported by major wind systems. Saharan dust transported in the equatorial trade winds belt, and North American soil material transported in the latitudes of the westerlies. Between these aeolian-dominated belts, along an axis from the Azores to Gibraltar and the Mediterranean, lies a zone of more radiogeniceNd(0) values in sediments and aerosols influenced by the recent volcanism of this region. Previously reported distributions of Nd isotopes in bottom-formed iron-manganese crusts and coatings, are similar to distributions of Nd isotope values in detrital sediments and aerosols. This suggests that the detrital fraction has undergone partial dissolution and desorption in the water column and/or in the pore waters of near-surface sediments, and that a significant proportion of Nd in the authigenic ferromanganese phases is derived from this source, and may therefore not represent strictly dissolved seawater Nd isotope ratios, useful as watermass tracers.

Distribution and evolution of carbon and nitrogen in Earth

In this paper we discuss the distribution, geochemical cycle, and evolution of CO 2 and N 2 in Earths degassed mantle, and atmosphere plus continental crust. We estimate the present distribution of CO 2 and N 2 in Earths degassed mantle based on amounts of He and Ar in the degassed mantle and observed outgassing ratios of CO2/He and N2/Ar at mid-ocean ridges. Estimated CO 2 in present degassed mantle is (1.8_ +9) x 1022 mol, representing (72 + 10)% of total degassable CO2, an amount far higher than fractions previously inferred for noble gases. This strongly suggests that most CO 2 has been recycled from Earths surface into the degassed mantle through subduction, which is consistent with many recent discussions. For N2, the estimated amount in the present mantle is ~ 2.5 × 1019 mol, representing ~ 12% of total degassable N 2. Recycling of N 2 back to the mantle is also inferred, but on a much smaller scale. A simple model for the outgassing and recycling of CO 2 and N 2 in Earth is presented. Outgassing is assumed to be via melt-vapor partitioning of volatiles. Recycling back into the mantle via subduction is assumed to be proportional to the mass of the volatile component in the crust. This simple model is consistent with all currently available constraints. Difficulties arise from the dependence of the recycling constant on time. Hence, no single evolution history can be obtained for CO 2 based on the available data. Model results tentatively point to a higher CO 2 content on Earths surface in the Archean and Proterozoic than at present. Important future constraints may come from records in sedimentary rocks, improved understanding of carbonate production, and better modeling of the recycling process.

Sr and Nd isotope systematics in fish teeth

High concentrations of Sr and Nd in fish teeth apatite (up to 2000 and 3800 ppm. respectively) make them relatively impervious to diagenetic overprints and allow high-precision analysis of87Sr/86 Sr and143Nd/144Nd of very small amounts of material.87Sr/86Sr ratios of 14 Cenozoic samples (< 55 Ma) from the Atlantic, Indian, and Pacific Oceans and the Caribbean Sea are close to the seawater value at the time of formation, suggesting their use for87Sr/86Sr stratigraphic dating. A compilation of published143Nd/144Nd data from recent Mn-nodules and marineFeMn deposits shows that the North-Central Pacific Ocean and the major portion of the Atlantic Ocean display distinctly different, narrow ranges in isotopic compositions. Between these two ares, the143Nd/144Nd of Mn-nodules vary systematically towards intermediate compositions in the Southern Oceans, reflecting the communication between the Atlantic and Pacific Oceans through the eastward moving Antarctic Circumpolar Current. The143Nd/144Nd of the 2–55 Ma old fish teeth analyzed are typically close to the range of143Nd/144Nd in Recent Mn-nodules and seawater for the respective ocean and, thus, indicate no significant change through this time period. The143Nd/144Nd of fish teeth from the North Pacific Ocean are highest during periods of high aeolian sedimentation. Since such dust probably introduces Nd with a low143Nd/144Nd, it is unlikely that the fish teeth contain any significant amounts of sediment-derived REEs. Our data suggest that fish teeth probably obtained their REE signature from seawater during early diagenesis, not from later diagenetic overprints. For this reason they may be used to study the communication between the paleo-oceans. The143Nd/144Nd of Caribbean Sea fish teeth is nearly constant through the past 8 Ma, precluding major influx of waters from the Pacific or the Atlantic Oceans, despite the evidence for faunal communication through the Isthmus of Panama prior to the Pliocene. The143Nd/144Nd of 33 Ma old fish teeth from the Giant Piston Core, deposited at near equatorial paleolatitudes, suggest that the Central and North Pacific Ocean did not receive sufficient Atlantic-derived REEs to alter its143Nd/144Nd. For this reason, a large-scale equatorial current through the Isthmus of Panama appears to be unlikely.

The longevity of the South Pacific isotopic and thermal anomaly

Abstract The South Pacific is anomalous in terms of the Sr, Nd, and Pb isotope ratios of its hot spot basalts, a thermally enhanced lithosphere, and possibly a hotter mantle. We have studied the Sr, Nd, and Pb isotope characteristics of 12 Cretaceous seamounts in the Magellans, Marshall and Wake seamount groups (western Pacific Ocean) that originated in this South Pacific Isotopic and Thermal Anomaly (SOPITA). The range and values of isotope ratios of the Cretaceous seamount data are similar to those of the island chains of Samoa, Tahiti, Marquesas and Cook/Austral in the SOPITA. These define two major mantle components suggesting that isotopically extreme lavas have been produced at SOPITA for at least 120 Ma. Shallow bathymetry, and weakened lithosphere beneath some of the seamounts studied suggests that at least some of the thermal effects prevailed during the Cretaceous as well. These data, in the context of published data, suggest: (1)|SOPITA is a long-lived feature, and enhanced heat transfer into the lithosphere and isotopically anomalous mantle appear to be an intrinsic characteristic of the anomaly. (2)|The less pronounced depth anomaly during northwesterly plate motion suggests that some of the expressions of SOPITA may be controlled by the direction of plate motion. Motion parallel to the alignment of SOPITA hot spots focusses the heat (and chemical input into the lithosphere) on a smaller cross section than oblique motion. (3)|The lithosphere in the eastern and central SOPITA appears to have lost its original depleted mantle characteristics, probably due to enhanced plume/lithosphere interaction, and it is dominated by isotopic compositions derived from plume materials. (4)|We speculate (following D.L. Anderson) that the origin of the SOPITA, and possibly the DUPAL anomaly is largely due to focussed subduction through long periods of the geological history of the earth, creating a heterogeneous distribution of recycled components in the lower mantle

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 μm) and large (> 53 μm) 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 μg/g and 5.8–11.8 μg/g, respectively). At 40 m, the Nd signature of large particles is significantly more negative (ϵNd(0) = -13.1 ± 0.2) than that of the small ones (ϵNd(0) = -11.7 ± 0.3 and the particles filtered deeper. Below 237 m, the ϵNd(0) values for small particles match the seawater, whereas the ϵNd(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 ϵNd(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.

Sr, Nd, and Pb isotope geochemistry of Tertiary and Quaternary alkaline volcanics from West Germany

Mantle-derived alkaline magmas from the Quaternary East and West Eifel volcanic fields (West Germany) and a range of basalts from various Tertiary Provinces in West Germany show considerable variation in their Nd and Sr isotopic compositions. East Eifel mafic magmas (basanites, leucitites) and derivatives thereof (tephrites, phonolites) are, with the exception of two nephelinites, very similar in87Sr/86Sr (0.70456–0.70472) but show a significant range in143Nd/144Nd (0.51263–0.51273). In contrast, West Eifel melilitite-nephelinites, leucitites, and melililites (F-group magmas), and olivine-nephelinites and basanites (ONB-group), define a steeply inclined array in a SrNd isotope variation diagram extending from 0.7039 to 0.7045 and from 0.51285 and 0.51267, for87Sr/86Sr and143Nd/144Nd respectively. ONB-magmas form a separate low87Sr/86Sr-high143Nd/144Nd group. These data combined with analyzed Tertiary lavas define an array extending from E-type MORB down to estimated bulk silicate earth values. Lead isotopic compositions of Eifel volcanics are rather radiogenic compared to other continental alkaline rocks and are positively and negatively correlated with143Nd/144Nd and87Sr/86Sr, respectively. An evaluation of possible contaminants demonstrates that crustal assimilation has not played a major role in determining the isotopic compositions of the lavas. Isotopic variations thus document heterogeneity on a variety of scales within the Eifel mantle source. Estimated source Sm/Nd ratios, in conjunction with measured isotopic compositions, suggest that mixing between enriched and depleted mantle materials, which occurred sometime during the past 1.1 Ga, resulted in the production of the heterogeneous proto-Eifel source. This “mixing” event may have occurred in conjunction with Tertiary magmatism in central Europe.

He, Pb, Sr and Nd isotope constraints on magma genesis and mantle heterogeneity beneath young Pacific seamounts

Pb, Sr and Nd isotope variations are correlated in diverse lavas erupted at small seamounts near the East Pacific Rise. Tholeiites are isotopically indistinguishable from MORB (206Pb/204Pb=18.1–18.5; 87Sr/86Sr=0.7023–0.7028; 143Nd/144Nd=0.51326-0.51308); associated alkali basalts always show more radiogenic Pb and Sr signatures (206Pb/204Pb=18.8–19.2; 87Sr/86Sr=0.7029–0.7031) and less radiogenic Nd (143Nd/144Nd=0.51289–0.51301). The isotopic variability covers ∼80% of the variability for Pacific MORB, due to the presence of small-scale heterogeneity in the underlying mantle. Isotope compositions also correlate with trace element ratios such as La/Sm. Tholeiites at these seamounts have 3He/4He between 7.8–8.7 RA(RA= atmospheric ratio), also indistinguishable from MORB. He trapped in vesicles of alkali basalts, released by crushing in vacuo, has low 3He/4He (1.2–2.6 R)Ain conjunction with low helium concentrations ([He]<5×10−8 ccSTP/g). In many cases post-eruptive radiogenic ingrowth has produced He isotope disequilibrium between vesicles and glass in the alkali basalts; subatmospheric 3He/4He ratios characterize the He dissolved in the glass which is released by melting the crushed powders. The narrow range of 3He/4He in the vesicles of the alkali basalts suggests that low 3He/4He is a source characteristic, but given their low [He] and high (U + Th), pre-eruptive radiogenic ingrowth cannot be excluded as a cause for low inherited 3He/4He ratios. Pb, Sr and Nd isotope compositions in lavas erupted at Shimada Seamount, an isolated volcano on 20 m.y. old seafloor at 17°N, are distinctly different from other seamounts in the East Pacific (206Pb/204Pb=18.8–19.0, 87Sr/ 86Sr≅0.7048 and 143Nd/144Nd≅0.51266). Relatively high 207Pb/204Pb (15.6–15.7) indicates ancient (>2 Ga) isolation of the source from the depleted upper mantle, similar to Dupal components which are more prevalent in the southern hemisphere mantle. 3He/4He at Shimada Seamount is between 3.9–4.8 RA. Because the helium concentrations range up to 1.5×10−6, the low 3He/4He can not be due to radiogenic accumulation of 4He in the magma for reasonable volcanic evolution times. The low 3He/4He may be due to the presence of “enriched” domains within the lithosphere with high (U + Th)/He ratios, possibly formed during its accretion near the ridge. Alternatively, the low 3He/4He may be an inherent characteristic of an enriched component in the mantle beneath the East Pacific. Collectively, the He-Pb-Sr-Nd isotope systematics at East Pacific seamounts suggest that the range of isotope compositions present in the mantle is more readily sampled by seamount and island volcanism than by axial volcanism. Beneath thicker lithosphere away from the ridge axis, smaller degrees of melting in the source regions are less efficient in averaging the chemical characteristics of small-scale heterogeneities.