Sune G. Nielsen

Thallium isotopic evidence for ferromanganese sediments in the mantle source of Hawaiian basalts

Ocean island basalts are generally thought to be the surface expression of mantle plumes, but the nature of the components in the source regions of such mantle plumes is a subject of long-standing debate. The lavas erupted at Hawaii have attracted particular attention, as it has been proposed that coupled 186Os and 187Os anomalies reflect interaction with the Earths metallic core. It has recently been suggested, however, that such variations could also result from addition of oceanic ferromanganese sediments to the mantle source of these lavas. Here we show that Hawaiian picrites with osmium isotope anomalies also exhibit pronounced thallium isotope variations, which are coupled with caesium/thallium ratios that extend to values much lower than commonly observed for mantle-derived rocks. This correlation cannot be created by admixing of core material, and is best explained by the addition of ferromanganese sediments into the Hawaii mantle source region. However, the lack of correlation between thallium and osmium isotopes and the high thallium/osmium ratios of ferromanganese sediments preclude a sedimentary origin for the osmium isotope anomalies, and leaves core–mantle interaction as a viable explanation for the osmium isotope variations of the Hawaiian picrites.

Early accretion of water in the inner solar system from a carbonaceous chondrite–like source

History recorded in asteroids water Astronomers know that interstellar water is abundantly available to young planetary systems—our blue planet collected (or accreted) plenty of it. Still, the details of waters movement in the inner solar system are elusive. Sarafian et al. measured water isotopes in meteorite samples from the asteroid Vesta for clues to the timing of water accretion. Their samples have the same isotopic fingerprint of volatiles as both Earth and carbonaceous chondrites, some of the most primitive meteorites. The findings suggest that Earth received most of its water relatively early from chondrite-like bodies. Science, this issue p. 623 Similar volatile isotopes of Earth and ancient meteorites point to an early accumulation of water for terrestrial bodies. Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth’s isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event.

Geochemical evidence for mélange melting in global arcs

Arc lavas form from melting of mélange rocks; sediment melts and slab-derived fluids are not major contributors. In subduction zones, sediments and hydrothermally altered oceanic crust, which together form part of the subducting slab, contribute to the chemical composition of lavas erupted at the surface to form volcanic arcs. Transport of this material from the slab to the overlying mantle wedge is thought to involve discreet melts and fluids that are released from various portions of the slab. We use a meta-analysis of geochemical data from eight globally representative arcs to show that melts and fluids from individual slab components cannot be responsible for the formation of arc lavas. Instead, the data are compatible with models that first invoke physical mixing of slab components and the mantle wedge, widely referred to as high-pressure mélange, before arc magmas are generated.

Petrogenesis of an early Archaean (3.4 Ga) norite dyke, Isua, West Greenland: evidence for early Archaean crustal recycling?

Abstract An unaltered norite cumulate dyke from the central part of the Isua area, West Greenland, has been studied in an attempt to determine the isotopic composition of early Archaean mantle. A mineral (plagioclase–orthopyroxene) and whole rock Sm–Nd isochron yields an age of 3413±57 Ma, which is slightly younger than a previously published U–Pb zircon SHRIMP age of 3512±6 Ma ( Nutman et al., 1997 ). While the mineralogy (opx+plag+ol+cr-sp) and mineral chemistry of the dyke (opx: En 90–70 , plag: An 73–54 , ol: Fo 90–87 , and Cr-sp: Cr 66–56 ) are consistent with crystallization from a near primary melt, a strong crustal signature characterizes the trace element and isotopic composition of the dyke i.e. (Nb/La) PM =0.20–0.25; e Nd =−1.3±1; (La/Yb) N =4.8–4.1. Hydrothermally altered samples up to 5 cm distant from a cross-cutting quartz-vein exhibit systematic changes in trace element and isotopic composition approaching the quartz-vein (a total variation of ∼2.5 e Nd -units; (La/Yb) N =5.5–4.1). This illustrates one danger of simple interpretations of isotope compositions of early Archaean whole rocks. Modeling the chemical and isotopic signature of the dyke either requires unrealistically large amounts (>20%) of crustal assimilation or introduction of smaller amounts (∼5%) of crustal material directly into a mantle reservoir prior to melting. Therefore, we suggest a petrogenetic model involving early Archaean crustal recycling processes.

Persistence of deeply sourced iron in the Pacific Ocean

Significance The vertical supply of dissolved Fe (iron) is insufficient compared with the physiological needs of marine phytoplankton in vast swathes of the open ocean. However, the relative importance of the main sources of “new” Fe to the ocean—continental mineral dust, hydrothermal exhalations, and sediment dissolution—and their temporal evolution are poorly constrained. By analyzing the isotopic composition of Fe in marine sediments, we find that much of the dissolved Fe in the central Pacific Ocean originated from hydrothermal and sedimentary sources thousands of meters below the sea surface. As such, these data underscore the vital role of the oceans’ physical mixing in determining if any deeply sourced Fe ever reaches the Fe-starved surface-dwelling biota. Biological carbon fixation is limited by the supply of Fe in vast regions of the global ocean. Dissolved Fe in seawater is primarily sourced from continental mineral dust, submarine hydrothermalism, and sediment dissolution along continental margins. However, the relative contributions of these three sources to the Fe budget of the open ocean remains contentious. By exploiting the Fe stable isotopic fingerprints of these sources, it is possible to trace distinct Fe pools through marine environments, and through time using sedimentary records. We present a reconstruction of deep-sea Fe isotopic compositions from a Pacific Fe−Mn crust spanning the past 76 My. We find that there have been large and systematic changes in the Fe isotopic composition of seawater over the Cenozoic that reflect the influence of several, distinct Fe sources to the central Pacific Ocean. Given that deeply sourced Fe from hydrothermalism and marginal sediment dissolution exhibit the largest Fe isotopic variations in modern oceanic settings, the record requires that these deep Fe sources have exerted a major control over the Fe inventory of the Pacific for the past 76 My. The persistence of deeply sourced Fe in the Pacific Ocean illustrates that multiple sources contribute to the total Fe budget of the ocean and highlights the importance of oceanic circulation in determining if deeply sourced Fe is ever ventilated at the surface.

Chalcophile behavior of thallium during MORB melting and implications for the sulfur content of the mantle

We present new laser ablation ICP-MS trace element concentration data for 28 elements in 97 mid-ocean ridge basalt (MORB) glasses that cover all major spreading centers as well as Tl concentration data for all mineral phases in five lherzolites from the Lherz massif, France. The ratio between the elements thallium (Tl) and cerium (Ce) is nearly constant in MORB, providing evidence that the depleted MORB mantle (DMM) has uniform Ce/Tl. Lherzolite mineral data reveal that sulfides are heterogeneous and contain between 23 and 430 ng/g of Tl while all other minerals contain Tl below the analytical detection limit of ∼1 ng/g. We argue that Tl in MORB is controlled by residual sulfide during mantle melting. To investigate the observed relationship between Tl and Ce, we conduct models of fractional mantle melting, which show that the constant Ce/Tl in MORB is only reproduced if the ratio between clinopyroxene and sulfide in the upper mantle varies by less than 10%. In addition, the rate of melting for these two phases must be nearly identical as otherwise melt depletion and refertilization processes would lead to Ce/Tl fractionation. These model results allow us to establish a relationship for the sulfur content of DMM: [S]DMM = SCSS × Mcpx /Rcpx, where SCSS is the sulfur concentration of a silicate melt at sulfide saturation, Rcpx is the melt reaction coefficient, and Mcpx is the modal abundance of clinopyroxene in the DMM. Using this equation, we calculate that the average upper mantle sulfur concentration is 195 ± 45 μg/g.

Thallium Isotopes and Their Application to Problems in Earth and Environmental Science

This paper presents an account of the advances that have been made to date on the terrestrial stable isotope geochemistry of thallium (Tl). High precision measurements of Tl isotope ratios were only developed in the late 1990s with the advent of MC-ICP-MS and therefore we currently only have limited knowledge of the isotopic behavior of this element. Studies have revealed that Tl isotopes, despite their heavy masses of 203 and 205 atomic mass units, can fractionate substantially, especially in the marine environment. The most fractionated reservoirs identified are ferromanganese sediments and low temperature altered of oceanic crust. These display a total isotope variation of about 35 e205Tl-units, which is over 50 times the analytical reproducibility of the measurement technique. The isotopic variation can be explained by invoking a combination of conventional mass dependent equilibrium isotope effects and the nuclear field shift isotope fractionation, but the specific mechanisms are still largely unaccounted for.

Constraining the rate of oceanic deoxygenation leading up to a Cretaceous Oceanic Anoxic Event (OAE-2: ~94 Ma)

A Tl isotope excursion preserved in shales leading up to OAE-2 provides evidence for progressive bottom water deoxygenation. The rates of marine deoxygenation leading to Cretaceous Oceanic Anoxic Events are poorly recognized and constrained. If increases in primary productivity are the primary driver of these episodes, progressive oxygen loss from global waters should predate enhanced carbon burial in underlying sediments—the diagnostic Oceanic Anoxic Event relic. Thallium isotope analysis of organic-rich black shales from Demerara Rise across Oceanic Anoxic Event 2 reveals evidence of expanded sediment-water interface deoxygenation ~43 ± 11 thousand years before the globally recognized carbon cycle perturbation. This evidence for rapid oxygen loss leading to an extreme ancient climatic event has timely implications for the modern ocean, which is already experiencing large-scale deoxygenation.

Analysis of high-precision vanadium isotope ratios by medium resolution MC-ICP-MS

We present and verify a new method to measure vanadium isotope ratios using a Thermo Scientific Neptune multi-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) operated in medium mass resolution mode. We collect masses 48 through 53 simultaneously using the L2, L1, Center, H1, H2 and H3 collectors. The Center cup is equipped with a 1012 Ω resistor, H1 is equipped with a 1010 Ω resistor, while the rest of the collectors have standard 1011 Ω resistors. Unlike previous low-resolution methods, the use of medium mass resolution (ΔM/M ∼ 4000) permits separation of V, Ti and Cr isotopes from all interfering molecular species representing combinations of C, N, O, S, Cl, and Ar. We show that the external reproducibility follows a power law function with respect to the number of V+ ions collected and achieve an external reproducibility of ±0.15‰ with total V+ ion beam intensities of ∼1 nA. The separation of interfering molecular species from the V mass spectrum reduces the V requirement for precise isotope data to as little as 200–300 ng V per analysis—a reduction of ∼90% compared with previous methods—making several low-V matrices amenable to V isotope analysis.

Potassium and uranium in the upper mantle controlled by Archean oceanic crust recycling

The Earth’s mantle is divided into separate reservoirs. From basaltic lavas erupted today we know that the upper mantle is depleted in incompatible trace elements, whereas ocean island basalts (OIB) sample a deep, more enriched reservoir. The two incompatible elements potassium (K) and uranium (U) display a different ratio to each other in lavas from the upper and lower mantle reservoirs. This is surprising because they do not fractionate substantially during melting of the mantle and therefore continuous extraction of melts from the upper mantle over Earth’s history cannot explain this disparity. Here a model is constructed in which U is insoluble during continental weathering in the anoxic conditions of the early Earth. This leads to recycling of oceanic crust with high K/U in the early Earth and low K/U after signifi cant oxidative weathering commenced ca. 2200 Ma. This concept reproduces the observed K/U in both the upper and lower mantle reservoirs as well as the continental crust. The model is also shown to account for the thorium/uranium ratios and thereby Pb (lead) isotope compositions in these reservoirs. Successful model solutions imply that (1) chemical weathering fl uxes to the oceans on the early Earth were signifi cantly higher than present day, and (2) that the majority of the OIB reservoir is younger than 2200 Ma.