Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots.

The oxygen fugacity (fO2) of the lower cratonic lithosphere influences diamond formation, melting mechanisms, and lithospheric evolution, but its redox evolution over time is unclear. We apply Cu isotopes (δ65Cu) of ~ 1.4 Ga lamproites and < 0.59 Ga silica-undersaturated alkaline rocks from the lithosphere-asthenosphere boundary (LAB) of the North Atlantic Craton to characterize fO2 and volatile speciation in their sources. The lamproites' low δ65Cu (-0.19 to -0.12‰) show that the LAB was metal-saturated with CH4 + H2O as the dominant volatiles during the Mesoproterozoic. The mantle-like δ65Cu of the < 0.59 Ga alkaline rocks (0.03 to 0.15‰) indicate that the LAB was more oxidized, stabilizing CO2 + H2O and destabilizing metals. The Neoproterozoic oxidation resulted in an increase of at least 2.5 log units in fO2 at the LAB. Combined with previously reported high fO2 in peridotites from the Slave, Kaapvaal, and Siberia cratonic roots, this oxidation might occur in cratonic roots globally.

Incipient carbonate melting drives metal and sulfur mobilization in the mantle.

We present results from high-pressure, high-temperature experiments that generate incipient carbonate melts at mantle conditions (~90 kilometers depth and temperatures between 750° and 1050°C). We show that these primitive carbonate melts can sequester sulfur in its oxidized form of sulfate, as well as base and precious metals from mantle lithologies of peridotite and pyroxenite. It is proposed that these carbonate sulfur-rich melts may be more widespread than previously thought and that they may play a first-order role in the metallogenic enhancement of localized lithospheric domains. They act as effective agents to dissolve, redistribute, and concentrate metals within discrete domains of the mantle and into shallower regions within Earth, where dynamic physicochemical processes can lead to ore genesis at various crustal depths.

Carbonate-rich crust subduction drives the deep carbon and chlorine cycles.

The flux balances of carbon and chlorine between subduction into the deep mantle and volcanic emissions into the atmosphere are crucial for the habitability of our planet1,2. However, pervasive loss of fluids from subducting slabs has been thought to cut off the delivery of both carbon and chlorine to the deep mantle owing to their high mobility under hydrous conditions3,4. Our new high-pressure experiments show that most carbonates (>75 wt%) in carbonate-rich crustal rocks-one of the main subducting carbon reservoirs-survive devolatilization and hydrous melting in cold and warm subduction zones, indicating that their subduction has driven the deep carbon cycle since the Mesoproterozoic. We found that KCl and NaCl, respectively, become stable phases crystallizing from hydrous carbonatite melts with low chlorine solubility in warm and hot subduction zones, resulting in the sequestration of chlorine in the solid residue in downwelling slabs. Accordingly, the subduction of carbonate-rich rocks facilitated highly effective recycling of both chlorine and carbon into the deep mantle at intermediate stages of Earth's history and led to declining atmospheric pCO2 and the formation of carbon-rich and chlorine-rich mantle reservoirs since the Mesoproterozoic. This period of optimal carbon and chlorine subduction may explain the ages of eclogitic diamonds and the formation of the HIMU mantle source.

Rapid quench piston cylinder apparatus: An improved design for the recovery of volatile-rich geological glasses from experiments at 0.5-2.5 GPa.

The accurate and precise determination of the compositions of silicate glasses formed from melts containing volatile components H2O and CO2 recovered from high-pressure, high-temperature experiments is essential to our understanding of geodynamic processes taking place within the planet. Silicate melts are often difficult to analyze chemically because the formation of quench crystals and overgrowths on silicate phases is rapid and widespread upon quenching of experiments, preventing the formation of glasses in low-SiO2 and volatile-rich compositions. Here, we present experiments conducted in a novel rapid quench piston cylinder apparatus on a series of partially molten low-silica alkaline rock compositions (lamproite, basanite, and calk-alkaline basalt) with a range of water contents between 3.5 and 10 wt %. Quench modification of the volatile-bearing silicate glasses is significantly reduced compared to those produced in older piston cylinder apparatuses. The recovered glasses are almost completely free of quench modification and facilitate the determination of precise chemical compositions. We illustrate significantly improved quench textures and provide an analytical protocol that recovers accurate chemical compositions from both poorly quenched and well-quenched silicate glasses.

The origin of the terrestrial noble-gas signature.

In the atmospheres of Earth and Mars, xenon is strongly depleted relative to argon, when compared to the abundances in chondritic meteorites. The origin of this depletion is poorly understood. Here we show that more than one weight per cent of argon may be dissolved in MgSiO(3) perovskite, the most abundant phase of Earth's lower mantle, whereas the xenon solubility in MgSiO(3) perovskite is orders of magnitude lower. We therefore suggest that crystallization of perovskite from a magma ocean in the very early stages of Earth's history concentrated argon in the lower mantle. After most of the primordial atmosphere had been lost, degassing of the lower mantle replenished argon and krypton, but not xenon, in the atmosphere. Our model implies that the depletion of xenon relative to argon indicates that perovskite crystallized from a magma ocean in the early history of Earth and perhaps also Mars.

Carbon solubility in olivine and the mode of carbon storage in the Earth's mantle.

The total amount of carbon in the atmosphere, oceans and other near-surface reservoirs is thought to be negligible compared to that stored in the Earth's mantle. Although the mode of carbon storage in the mantle is largely unknown, observations of microbubbles on dislocations in minerals from mantle xenoliths has led to the suggestion that carbon may be soluble in silicates at high pressure. Here we report measurements of carbon solubility in olivine, the major constituent of the upper mantle, at pressures up to 3.5 GPa. We have found that, contrary to previous expectations, carbon solubility in olivine is exceedingly low--of the order of 0.1 to 1 parts per million by weight. Together with similar data for pyroxenes, garnet and spinel, we interpret this to imply that most carbon must be present as a separate phase in the deeper parts of the upper mantle, probably as a carbonate phase. Large-scale volcanic eruptions tapping such a carbonate-bearing mantle reservoir might therefore rapidly transfer large amounts of carbon dioxide into the atmosphere, consistent with models that link global mass extinctions to flood basalt eruptions via a sudden increase in atmospheric carbon dioxide levels.