Nickel and helium evidence for melt above the core–mantle boundary

High ^(3)He/^(4)He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source. This helium source may have been isolated at the core–mantle boundary region since Earth’s accretion. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth; if so, it is now either a primitive refugium at the core–mantle boundary or is distributed throughout the lower mantle. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high ^(3)He/^(4)He. We propose that a less-degassed nickel-rich source formed by core–mantle interaction during the crystallization of a melt-rich layer or basal magma ocean, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core–mantle boundary

The role of an H₂O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55-68 wt%), Na2O (~25-33 wt%) and K2O (~5-13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 - 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and )Nd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and )Nd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment)

Noble gases recycled into the mantle through cold subduction zones

Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole – a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5–35% and 60–80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior

Noble gases recycled into the mantle through cold subduction zones

Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole – a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5–35% and 60–80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior

Noble gas transport into the mantle facilitated by high solubility in amphibole

The chemical evolution of both the Earth’s atmosphere and mantle can be traced using noble gases. Their abundance in mantle and atmosphere is largely determined by a balance between the flux of noble gases from the Earth’s interior through magmatism, and the recycling of noble gases from the atmosphere back into the mantle at subduction zones. The flux of noble gases back into the mantle has long been thought to be negligible. Analyses of samples from the mantle now suggest that this recycling flux is more significant, but the mechanism is unclear. Here we present high-pressure experimental measurements that demonstrate high solubility of noble gases in amphibole, an important hydrous mineral in altered oceanic crust. Noble gas solubility correlates with the concentration of unoccupied A-sites, sites within the amphibole lattice structure that are constituted by a pair of opposing tetrahedra rings. We conclude that A-sites are energetically favourable locations for noble gas dissolution in amphibole that could allow recycling of noble gases into the mantle by subduction of altered oceanic crust. As many hydrous minerals in subducting slabs, such as serpentine and chlorite, have lattice structures similar to the A-site in amphibole, we suggest that these minerals may provide even more significant recycling pathways

Helium isotopic evidence for episodic mantle melting and crustal growth

The timing of formation of the Earth’s continental crust is the subject of a long-standing debate, with models ranging from early formation with little subsequent growth, to pulsed growth, to steadily increasing growth. But most models do agree that the continental crust was extracted from the mantle by partial melting. If so, such crustal extraction should have left a chemical fingerprint in the isotopic composition of the mantle. The subduction of oceanic crust and subsequent convective mixing, however, seems to have largely erased this record in most mantle isotopic systems (for example, strontium, neodymium and lead). In contrast, helium is not recycled into the mantle because it is volatile and degasses from erupted oceanic basalts. Therefore helium isotopes may potentially preserve a clearer record of mantle depletion than recycled isotopes. Here I show that the spectrum of 4He/3He ratios in ocean island basalts appears to preserve the mantle’s depletion history, correlating closely with the ages of proposed continental growth pulses. The correlation independently predicts both the dominant 4He/3He peak found in modern mid-ocean-ridge basalts, as well as estimates of the initial 4He/3He ratio of the Earth. The correspondence between the ages of mantle depletion events and pulses of crustal production implies that the formation of the continental crust was indeed episodic and punctuated by large, potentially global, melting events. The proposed helium isotopic evolution model does not require a primitive, undegassed mantle reservoir, and therefore is consistent with whole mantle convection

Deposition of 1.88-billion-year-old iron formations as a consequence of rapid crustal growth

Iron formations are chemical sedimentary rocks comprising layers of iron-rich and silica-rich minerals whose deposition requires anoxic and iron-rich (ferruginous) sea water. Their demise after the rise in atmospheric oxygen by 2.32 billion years (Gyr) ago has been attributed to the removal of dissolved iron through progressive oxidation or sulphidation of the deep ocean. Therefore, a sudden return of voluminous iron formations nearly 500 million years later poses an apparent conundrum. Most late Palaeoproterozoic iron formations are about 1.88 Gyr old and occur in the Superior region of North America. Major iron formations are also preserved in Australia, but these were apparently deposited after the transition to a sulphidic ocean at 1.84 Gyr ago that should have terminated iron formation deposition, implying that they reflect local marine conditions. Here we date zircons in tuff layers to show that iron formations in the Frere Formation of Western Australia are about 1.88 Gyr old, indicating that the deposition of iron formations from two disparate cratons was coeval and probably reflects global ocean chemistry. The sudden reappearance of major iron formations at 1.88 Gyr ago—contemporaneous with peaks in global mafic–ultramafic magmatism, juvenile continental and oceanic crust formation, mantle depletion and volcanogenic massive sulphide formation—suggests deposition of iron formations as a consequence of major mantle activity and rapid crustal growth.Our findings support the idea that enhanced submarine volcanism and hydrothermal activity linked to a peak in mantle melting released large volumes of ferrous iron and other reductants that overwhelmed the sulphate and oxygen reservoirs of the ocean, decoupling atmospheric and seawater redox states, and causing the return of widespread ferruginous conditions. Iron formations formed on clastic-starved coastal shelves where dissolved iron upwelled and mixed with oxygenated surface water. The disappearance of iron formations after this event may reflect waning mafic–ultramafic magmatism and a diminished flux of hydrothermal iron relative to seawater oxidants