D. J. Frost

The redox state of the mantle during and just after core formation

Siderophile elements are depleted in the Earths mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal–silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe2O3 content of the mantle that probably occurred in the first 1 Ga of the Earths history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe2O3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal–silicate partition coefficients for O and Si, we have modelled core–mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe2O3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe2O3 during core formation, FeO would have disproportionated to produce Fe2O3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe2O3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

Lattice thermal conductivity of lower mantle minerals and heat flux from Earth’s core

The amount of heat flowing from Earth’s core critically determines the thermo-chemical evolution of both the core and the lower mantle. Consisting primarily of a polycrystalline aggregate of silicate perovskite and ferropericlase, the thermal boundary layer at the very base of Earth’s lower mantle regulates the heat flow from the core, so that the thermal conductivity (k) of these mineral phases controls the amount of heat entering the lowermost mantle. Here we report measurements of the lattice thermal conductivity of pure, Al-, and Fe-bearing MgSiO3 perovskite at 26 GPa up to 1,073 K, and of ferropericlase containing 0, 5, and 20% Fe, at 8 and 14 GPa up to 1,273 K. We find the incorporation of these elements in silicate perovskite and ferropericlase to result in a ∼50% decrease of lattice thermal conductivity relative to the end member compositions. A model of thermal conductivity constrained from our results indicates that a peridotitic mantle would have k = 9.1 ± 1.2 W/m K at the top of the thermal boundary layer and k = 8.4 ± 1.2 W/m K at its base. These values translate into a heat flux of 11.0 ± 1.4 terawatts (TW) from Earth’s core, a range of values consistent with a variety of geophysical estimates.

Ordering of hydroxyl defects in hydrous wadsleyite (β-Mg2SiO4)

Abstract Samples of wadsleyite (β-Mg2SiO4) containing a range of dissolved water concentrations up to 1.5 wt% were synthesized at 1300 °C and 15 GPa. The samples were studied using 1H MAS NMR and FTIR spectroscopy to determine the ordering of OH within the structure. As the 1H NMR chemical shift and O-H stretching frequency are both known to be correlated with the O-H···O distances in silicate and other materials, the spectroscopic data were compared with O···O distances calculated from the published crystal structure of hydrous wadsleyite. Using this approach we show that the hydroxyl ions in wadsleyite containing 0.8-1.5 wt% H2O are highly disordered, occupying at least 14 of the 17 possible O-H···O environments, including some with strong hydrogen bonding. In contrast, for low water concentrations (<0.4 wt%) the hydroxyl ions are less disordered, with four environments being much more abundant than the others are. Three of these environments appear to involve protonation of O1, in agreement with most previous suggestions, and the fourth is probably O2-H···O2. It is probable that the degree of disorder will increase with increasing temperature, so it should be taken into account when predicting phase equilibria involving hydrous wadsleyite and when extrapolating data on density and elastic properties from room temperature measurements.

Evidence for a Fe3+-rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data

The chemical composition of Earth’s lower mantle can be constrained by combining seismological observations with mineral physics elasticity measurements. However, the lack of laboratory data for Earth’s most abundant mineral, (Mg,Fe,Al)(Al,Fe,Si)O3 bridgmanite (also known as silicate perovskite), has hampered any conclusive result. Here we report single-crystal elasticity data on (Al,Fe)-bearing bridgmanite (Mg0.9Fe0.1Si0.9Al0.1)O3 measured using high-pressure Brillouin spectroscopy and X-ray diffraction. Our measurements show that the elastic behaviour of (Al,Fe)-bearing bridgmanite is markedly different from the behaviour of the MgSiO3 endmember. We use our data to model seismic wave velocities in the top portion of the lower mantle, assuming a pyrolitic mantle composition and accounting for depth-dependent changes in iron partitioning between bridgmanite and ferropericlase. We find excellent agreement between our mineral physics predictions and the seismic Preliminary Reference Earth Model down to at least 1,200 kilometres depth, indicating chemical homogeneity of the upper and shallow lower mantle. A high Fe3+/Fe2+ ratio of about two in shallow-lower-mantle bridgmanite is required to match seismic data, implying the presence of metallic iron in an isochemical mantle. Our calculated velocities are in increasingly poor agreement with those of the lower mantle at depths greater than 1,200 kilometres, indicating either a change in bridgmanite cation ordering or a decrease in the ferric iron content of the lower mantle.

Incorporation of Fe and Al in MgSiO3 perovskite: An investigation by 27Al and 29Si NMR spectroscopy

Abstract We present 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg1-xFex)(Si1-xAlx)O3 (x = 0.01, 0.025, and 0.05). 27Al MAS-NMR spectra of the perovskite samples indicate that Al3+ replaces both Si4+ in the octahedral site and Mg2+ in the larger 12-coordinated site. NMR signal loss caused by paramagnetic interactions is often a severe complication when performing NMR on materials containing Fe2+,3+; however, careful measurement of signal loss and comparison to total Fe content in these samples sheds light on the nature of Al and Fe incorporation. NMR signal loss for the pyroxenes is linearly related to total Fe content as would be expected in the case of uncorrelated substitution of randomly distributed Al and Fe. However, 27Al signal loss for the perovskite samples increases only slightly between samples with x = 0.01 and 0.025 indicating similar coordination of Al by Fe and non-random distribution. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe2+ and Fe3+ concentration at which useful NMR data can be obtained for this system.

A new multi-anvil press employing six independently acting 8 MN hydraulic rams

A new large volume multi-anvil system which employs six independently acting hydraulic rams with independent oil pressurization systems has been developed for high pressure and temperature experiments. The six 8 MN hydraulic rams approach at right angles inside a composite steel plate frame and can each advance a square faceted anvil of either hardened steel or tungsten carbide. The position of each anvil can be measured relative to the frame of the press to a precision of 0.1 μ m. The press is designed to perform both deformation experiments using cubic ceramic pressure media and experiments employing eight inner cubic anvils to compress an octahedral pressure medium. During compression, the position of each anvil relative to the press frame can be precisely measured and controlled independently, thus ensuring a high level of symmetry in the compressive stress environment. The highly cubic compressive regime provides an optimal environment for the use of inner sintered diamond cubic anvils, which can potentially obtain pressures above 50 GPa. The large loading capacity (24 MN) allows larger cubic pressure media to be used at higher pressures than conventional systems.

Melting experiments of a chondritic meteorite between 16 and 25 GPa: Implication for Na/K fractionation in a primitive chondritic Earth's mantle

Melting experiments at high-pressure and temperature were conducted at pressures from 16 to 25 GPa using chondritic starting material but with slightly enhanced Na- and K-contents while keeping the chondritic Na/K ratio constant. The experiments revealed that majorite garnet contains enhanced concentrations of Na (3.75-4.71 wt.% Na2O) and moderately enhanced concentrations of K (0.3-0.46 wt.% K2O) from 16 to 20 GPa and it is therefore the Na- and K-bearing phase at this pressure range. The Na2O/K2O ratio (8-15) in garnet is close to the initial chondritic ratio (7) of the starting material, which might indicate that at these pressures only little Na/K fractionation takes place. At pressures above 21 GPa, the high-pressure phases enriched in Na include majorite garnet (3.08-6.22 wt.% Na2O), magnesiowustite (1.83-3.3 wt.% Na2O), and (Mg,Fe)SiO3-perovskite (0.43-1.25 wt.% Na2O). These phases contain only small amounts of K ( 21 GPa should have taken place in the early accretional period in the primitive chondritic mantle of the Earth.

In situ determination of the spinel-post-spinel transition in Fe3O4 at high pressure and temperature by synchrotron X-ray diffraction

Abstract The position of the spinel-post-spinel phase transition in Fe3O4 has been determined in pressuretemperature space by in situ measurements using a multi-anvil press combined with white synchrotron radiation. Pressure measurement using the equation of state for MgO permitted pressure changes to be monitored at high temperature. The phase boundary was determined by the first appearance of diffraction peaks of the high-pressure polymorph (h-Fe3O4) during pressure increase and the disappearance of these peaks on pressure decrease along several isotherms. We intersected the phase boundary over the temperature interval of 700-1400 °C. The boundary is linear and nearly isobaric, with a slightly positive slope. Post-experiment investigation by TEM confirms that the reverse reaction from h-Fe3O4 to magnetite during decompression leads to the formation of microtwins on the (311) plane in the newly formed magnetite. Observations made during the phase transition suggest that the transition has a pseudomartensitic character, explaining in part why magnetite persists at conditions well within the stability field of h-Fe3O4, even at high temperatures. This study emphasizes the utility of studying phase transitions in situ at simultaneously high temperatures and pressures since the reaction kinetics may not be favorable at room temperature.

Phase relations of MgFe2O4 at conditions of the deep upper mantle and transition zone

Abstract Phase relations of magnesioferrite (MgFe2O4) have been studied between 8 and 18 GPa and 1000–1600 °C using multi-anvil experiments. At 8–10 GPa and 900–1200 °C, MgFe2O4 breaks down to Fe2O3+MgO. At higher temperatures, a new phase appears along with Fe2O3. Although this new phase is unquenchable, EPMA and TEM data point to a composition with Mg5Fe2O8 or Mg4Fe2O7 stoichiometry. Depending on pressure and temperature, other stoichiometries also appear to be stable together with Fe2O3. In terms of pressure, the stability field of the unquenchable phases + hematite widens with increasing temperature to 3 ± 1 GPa at ~1400 °C, and then narrows to ~1 GPa at 1600 °C. The recoverable assemblage of Mg2Fe2O5+Fe2O3 becomes stable between 11–13 GPa. The Mg2Fe2O5+Fe2O3 assemblage is stable up to at least 18 GPa at 1300 °C without any evidence of a hp-MgFe2O4 phase. In addition, hematite plays an important role in the phase relations of MgFe2O4 by being present over a wide range in pressure and temperature together with a Mg-rich Fe-oxide. Interestingly, hematite incorporates variable amounts of Mg whereby its concentration appears to be a function of temperature. This experimental study has implications for interpreting inclusions in natural diamonds where magnesioferrite occurs by placing a maximum pressure stability on the formation of this phase. Through these inclusions, it also provides constraints on diamond formation and their subsequent evolution prior to eruption. For example, the occasional observation of nano-sized magnesioferrite within (Mg,Fe)O inclusions must have either formed from a high-pressure precursor phase with a different stoichiometry at transition zone or upper lower mantle conditions, or it exsolved directly from the host (Mg,Fe)O under upper mantle conditions (i.e., <9–10 GPa). Since several studies report various non-silicate inclusions with simple oxide compositions, including magnesioferrite, magnetite, or ferropericlase, such inclusions provide evidence for variable redox conditions at the time of entrapment.

Synthesis of a Nitrogen-Stabilized Hexagonal Re3ZnNx Phase Using High Pressures and Temperatures

High pressure can induce profound changes in solids. A significant barrier to new alloys and ceramics, however, is that targeted starting materials may not react with each other, even with the help of pressure. We use nitrogen, in a new capacity, to incorporate two otherwise unreactive elements, Re and Zn, in the same structure when pressure alone does not suffice, without nitrogen altering the resulting backbone structure. Synthesis experiments up to 20 GPa and 1800 K show that while no Re-Zn alloy or solid solution is formed, a novel Re(3)ZnN(x) ordered solid solution is formed, at 20 GPa, with nitrogen occupying Re-coordinated cages. We put forth that unlike pure Re(3)Zn, our novel hexagonal Re(3)ZnN(x) structure is stabilized by nitrogen bond formation with rhenium. Pressure lifts the pronounced ambient Zn anisotropy, making it more compatible with Re and likely facilitating incorporation of the structure-stabilizing nitrogen anion. This methodology and result denote further options for removing impasses to material preparation, thus opening new avenues for synthesis. These can also be pursued with other ions including carbon, hydrogen, and oxygen, in addition to nitrogen.