Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume

Highest terrestrial 3He/4He credibly from the core

The observation that many lavas associated with mantle plumes have higher 3He/4He ratios than the upper convecting mantle underpins geophysical, geodynamic and geochemical models of Earth's deep interior. High 3He/4He ratios are thought to derive from the solar nebula or from solar-wind-irradiated material that became incorporated into Earth during early planetary accretion. Traditionally, this high-3He/4He component has been considered intrinsic to the mantle, having avoided outgassing caused by giant impacts and billions of years of mantle convection1-4. Here we report the highest magmatic 3He/4He ratio(67.2 ± 1.8 times the atmospheric ratio) yet measured in terrestrial igneous rocks, in olivines from Baffin Island lavas. We argue that the extremely high-3He/4He helium in these lavas might derive from Earth's core5-9. The viability of the core hypothesis relaxes the long-standing constraint-based on noble gases in lavas associated with mantle plumes globally-that volatile elements from the solar nebula have survived in the mantle since the early stages of accretion.

Boron and oxygen isotope evidence for recycling of subducted components over the past 2.5 Gyr

Evidence for the deep recycling of surficial materials through the Earth's mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and location of parent-daughter fractionation are not known. Conversely, while stable isotopes can provide irrefutable evidence for low-temperature fractionation, their range in most unaltered oceanic basalts is limited and the age of any variation is unconstrained. Here we show that delta(18)O ratios in basalts from the Azores are often lower than in pristine mantle. This, combined with increased Nb/B ratios and a large range in delta(11)B ratios, provides compelling evidence for the recycling of materials that had undergone fractionation near the Earth's surface. Moreover, delta(11)B is negatively correlated with (187)Os/(188)Os ratios, which extend to subchondritic values, constraining the age of the high Nb/B, (11)B-enriched endmember to be more than 2.5 billion years (Gyr) old. We infer this component to be melt- and fluid-depleted lithospheric mantle from a subducted oceanic plate, whereas other Azores basalts contain a contribution from approximately 3-Gyr-old melt-enriched basalt. We conclude that both components are most probably derived from an Archaean oceanic plate that was subducted, arguably into the deep mantle, where it was stored until thermal buoyancy caused it to rise beneath the Azores islands approximately 3 Gyr later.

Synthesis and characterization of a new ternary imide-Li2Ca(NH)2

The ternary imide Li(2)Ca(NH)2 was successfully synthesized by dehydrogenating a mixture of LiNH(2) and CaH(2) at a molar ratio of 2:1 in a stream of purified argon at 300 degrees C. A powder X-ray diffraction measurement revealed that Li(2)Ca(NH)2 was of the trigonal anti-La(2)O(3) structure (space group Pm1) with lattice constants of a = 3.5664(3)A and c = 5.9540(8) A. Ca occupied the 1b site (0, 0, 1/2), Li occupied the 2d site (1/3, 2/3, 0.8841(22)), and N occupied the 2d site (1/3, 2/3, 0.2565(15)). Nuclear magnetic resonance and X-ray absorption fine structure analyses demonstrated that each Li ion was coordinated with four imide ions and each Ca ion was coordinated with six imide ions.

A Mechanism for Non-stoichiometry in the Lithium Amide/Lithium Imide Hydrogen Storage Reaction

We demonstrate, through structural refinement from synchrotron X-ray diffraction data, that the mechanism of the transformation between lithium amide and lithium imide during hydrogen cycling in the important Li-N-H hydrogen storage system is a bulk reversible reaction that occurs in a non-stoichiometric manner within the cubic anti-fluorite-like Li-N-H structure.

Hydrogen Release from Mg(NH2)2−MgH2 through Mechanochemical Reaction

A total of 7.4 wt % of hydrogen was released from the mixture of magnesium amide and magnesium hydride at a molar ratio of 1:2 by mechanical ball milling. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) characterizations along with the amount of hydrogen released at different stages of ball milling reveal that magnesium imide was first formed in the reaction. The imide then reacted continuously with magnesium hydride and was converted to magnesium nitride and hydrogen. Thermodynamic calculation shows that the hydrogen desorption is a mild endothermic reaction with the standard enthalpy change of about 3.5 kJ/mol of H2.

Physiochemical Pathway for Cyclic Dehydrogenation and Rehydrogenation of LiAlH4

A five-step physiochemical pathway for the cyclic dehydrogenation and rehydrogenation of LiAlH4 from Li3AlH6, LiH, and Al was developed. The LiAlH4 produced by this physiochemical route exhibited excellent dehydrogenation kinetics in the 80-100 degrees C range, providing about 4 wt % hydrogen. The decomposed LiAlH4 was also fully rehydrogenated through the physiochemical pathway using tetrahydrofuran (THF). The enthalpy change associated with the formation of a LiAlH4.4THF adduct in THF played the essential role in fostering this rehydrogenation from the Li3AlH6, LiH, and Al dehydrogenation products. The kinetics of rehydrogenation was also significantly improved by adding Ti as a catalyst and by mechanochemical treatment, with the decomposition products readily converting into LiAlH4 at ambient temperature and pressures of 4.5-97.5 bar.

Quantum Delocalization of Hydrogen in the Li2NH Crystal

By mapping out potential energy surfaces from density-functional theory (DFT) and solving a protonic Schrödinger equation, we find that the H atom in a unit cell of the Li2NH crystal shows remarkably strong quantum behavior, leading to the delocalization of H over six octahedral sites around each N. This can be rationalized in terms of rapid coherent tunneling among these equivalent octahedral sites. Structural and dynamical consequences of our finding are discussed. Since the Li-N-H compounds are considered promising candidates for H-storage, understanding of these fundamental properties will be useful toward improving the performance of the material.

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.

Tungsten isotope evidence that mantle plumes contain no contribution from the Earth's core

Osmium isotope ratios provide important constraints on the sources of ocean-island basalts, but two very different models have been put forward to explain such data. One model interprets (187)Os-enrichments in terms of a component of recycled oceanic crust within the source material. The other model infers that interaction of the mantle with the Earth's outer core produces the isotope anomalies and, as a result of coupled (186)Os-(187)Os anomalies, put time constraints on inner-core formation. Like osmium, tungsten is a siderophile ('iron-loving') element that preferentially partitioned into the Earth's core during core formation but is also 'incompatible' during mantle melting (it preferentially enters the melt phase), which makes it further depleted in the mantle. Tungsten should therefore be a sensitive tracer of core contributions in the source of mantle melts. Here we present high-precision tungsten isotope data from the same set of Hawaiian rocks used to establish the previously interpreted (186)Os-(187)Os anomalies and on selected South African rocks, which have also been proposed to contain a core contribution. None of the samples that we have analysed have a negative tungsten isotope value, as predicted from the core-contribution model. This rules out a simple core-mantle mixing scenario and suggests that the radiogenic osmium in ocean-island basalts can better be explained by the source of such basalts containing a component of recycled crust.