Zhixue Du

High‐pressure melting of MgO from (Mg,Fe)O solid solutions

Magnesium oxide (MgO) is a significant component of planetary interiors, particularly Earths mantle and other rocky planets within and beyond our solar system; thus its high-pressure, high-temperature behavior is important to understanding the thermochemical evolution of planets. Laser-heated diamond-anvil cell (DAC) experiments on (Mg,Fe)O ferropericlase up to ~40 GPa show that previous DAC experiments on MgO melting are too low, while previous multi-anvil experiments yield melting temperatures too high. Instead, our quasi-static experimental results are consistent with recent ab initio predictions as well as dynamic shock measurements. Extrapolated to the core-mantle boundary (CMB) of the Earth, MgO is expected to melt at ~8000 ± 500 K, much greater than expected geotherm temperatures.

Mapping temperatures and temperature gradients during flash heating in a diamond-anvil cell

Here, we couple two-dimensional, 4-color multi-wavelength imaging radiometry with laser flash heating to determine temperature profiles and melting temperatures under high pressures in a diamond-anvil cell. This technique combines the attributes of flash heating (e.g., minimal chemical reactions, thermal runaway, and sample instability), with those of multi-wavelength imaging radiometry (e.g., 2D temperature mapping and reduction of chromatic aberrations). Using this new technique in conjunction with electron microscopy makes a powerful tool to determine melting temperatures at high pressures generated by a diamond-anvil cell.

Efficient graphite ring heater suitable for diamond-anvil cells to 1300 K

In order to generate homogeneous high temperatures at high pressures, a ring-shaped graphite heater has been developed to resistively heat diamond-anvil cell (DAC) samples up to 1300 K. By putting the heater in direct contact with the diamond anvils, this graphite heater design features the following advantages: (1) efficient heating: sample can be heated to 1300 K while the DAC body temperature remains less than 800 K, eliminating the requirement of a special alloy for the DAC; (2) compact design: the sample can be analyzed with in situ measurements, e.g., x-ray, optical, and electrical probes are possible. In particular, the side access of the heater allows for radial x-ray diffraction (XRD) measurements in addition to traditional axial XRD.

The influence of wavelength-dependent absorption and temperature gradients on temperature determination in laser-heated diamond-anvil cells

In situ temperature measurements in laser-heated diamond-anvil cells (LHDACs) are among the most fundamental experiments undertaken in high-pressure science. Despite its importance, few efforts have been made to examine the alteration of thermal radiation spectra of hot samples by wavelength-dependent absorption of the sample itself and temperature gradients within the sample and their influence on temperature measurements while laser heating. In this study, we take (Mg, Fe)O ferropericlase as an example to evaluate the effects of these two factors. Iron-rich ferropericlase shows strong wavelength-dependent absorption in the wavelength range used to determine temperature, which, together with temperature gradients can account for largely aliased apparent temperatures in some experiments obtained by Wien fitting of detected thermal radiation intensities (e.g., an offset of ∼700 K for a 3300 K melting temperature). In general, wavelength-dependent absorption and temperature gradients of samples are two key ...

Early episodes of high-pressure core formation preserved in plume mantle

The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth’s earliest stages of formation. Xe isotopic anomalies have been linked to degassing during accretion, but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated 3He/4He ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts. Portions of mantle involved in early high-pressure core formation would also be rich in FeO, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs with high 3He/4He ratios.

Using stepped anvils to make even insulation layers in laser-heated diamond-anvil cell samples

We describe a method to make even insulation layers for high-pressure laser-heated diamond-anvil cell samples using stepped anvils. The method works for both single-sided and double-sided laser heating using solid or fluid insulation. The stepped anvils are used as matched pairs or paired with a flat culet anvil to make gasket insulation layers and not actually used at high pressures; thus, their longevity is ensured. We compare the radial temperature gradients and Soret diffusion of iron between self-insulating samples and samples produced with stepped anvils and find that less pronounced Soret diffusion occurs in samples with even insulation layers produced by stepped anvils.

Experimental Constraints on Ferropericlase (Mg, Fe)O Melt Viscosity Up to 70 GPa: Experimental Constraints on Ferropericlase (Mg, Fe)O Melt Viscosity Up to 70 GPa

During Earth’s accretion, Earth’s mantle is expected to have been a magma ocean due to large impacts. As such, properties of molten mantle materials are key to understanding Earth’s thermochemical evolution. However, due to experimental challenges, transport properties at lower mantle pressures, particularly viscosity, are poorly constrained for mantle melts. In this study, we use quenched dendritic textures to estimate melt viscosities at high pressures for (Mg, Fe)O ferropericlase, one of the major components of the mantle. We find that the viscosity of (Mg, Fe)O melt near liquidus temperatures is ~10 –10 2 Pa s over the pressure range of 3–70 GPa, which is ~1–2 orders of magnitude lower than previous results for Si-rich melts at similar conditions. This may have implications for magma ocean cooling and thermochemical evolution of the mantle.