Much of the lunar crust is monomineralic, comprising >98% plagioclase. The prevailing model argues the crust accumulated as plagioclase floated to the surface of a solidifying lunar magma ocean ...(LMO). Whether >98% pure anorthosites can form in a flotation scenario is debated. An important determinant of the efficiency of plagioclase fractionation is the viscosity of the LMO liquid, which was unconstrained. Here we present results from new experiments conducted on a late LMO‐relevant ferrobasaltic melt. The liquid has an exceptionally low viscosity of
0.22−0.19+0.11 to
1.45−0.82+0.46 Pa s at experimental conditions (1,300–1,600°C; 0.1–4.4 GPa) and can be modeled by an Arrhenius relation. Extrapolating to LMO‐relevant temperatures, our analysis suggests a low viscosity LMO would form a stratified flotation crust, with the oldest units containing a mafic component and with very pure younger units. Old, impure crust may have been buried by lower crustal diapirs of pure anorthosite in a serial magmatism scenario.
Key Points
New measurements establish the low viscosity of a lunar magma ocean‐relevant ferrobasalt at lunar P‐T conditions (0.22–1.45 Pa s)
Plagioclase would float on a low‐viscosity lunar magma ocean to produce a crust with impure older units and very pure younger units
Very pure crust in shallow impact basins and the lunar highlands is consistent with resurfacing of the Moon by lower crustal diapirs
Plain Language Summary
This manuscript reports the first experimental measurements of the viscosity of a late lunar magma ocean liquid. Sample studies and remote sensing suggest the Moon's crust is effectively composed of one mineral (plagioclase). The crust is thought to have formed by flotation of buoyant plagioclase on a crystallizing lunar magma ocean; whether such a pure crust can form in a flotation scenario is debated. The purity of a flotation crust crucially depends on the viscosity of the magma ocean which was previously unconstrained. Experiments were conducted at high pressure‐temperature conditions at Argonne National Laboratory to characterize viscosity. Measured viscosities are as low as or lower than geologically‐relevant silicate melts measured previously. In a two part analysis, we consider the effect of the low viscosity lunar magma ocean on the purity of the crust. We conclude that the oldest flotation crust will contain a significant mafic component, while younger portions of the flotation crust will be pure, resulting in a stratified crust with mafic poor and rich units. The high purity of the present‐day lunar crust surface can best be explained by ancient resurfacing of the crust by the rise of buoyant lower crustal diapirs.
Preferential iron partitioning into melt during melting and crystallization of lower mantle minerals – bridgmanite and ferropericlase – can play a critical role in our understanding of the origin of ...the early Earth and its evolution to form chemically and seismically distinct regions in the present lowermost mantle. Of particular interest is the consequence of iron spin crossover in ferropericlase on the physical and chemical properties of the molten materials under relevant pressure–temperature (P–T) conditions of the lowermost mantle. However, the spin crossover in liquid (Mg, Fe)O and its effects on melting curves, iron partitioning, melt density – and thus the evolution of an early basal magma ocean – remain poorly studied. Here we conducted high P–T melting experiments on ferropericlase with a starting composition of (Mg0.86Fe0.14)O using synchrotron X-ray diffraction up to ∼120 GPa and ∼5400 K in laser-heated diamond anvil cells, together with chemical analyses on quenched samples using focused ion beam and energy dispersive spectroscopy technique. An ideal solid solution model could be satisfactorily used to fit the experimental data of the liquidus and solidus of (Mg, Fe)O for pure high-spin (HS, below ∼83 GPa), and low-spin (LS, above ∼120 GPa) states, respectively. The experimental solidus and liquidus at 99 GPa and ∼4000–5200 K strongly deviate from ideal solid solution behavior for pure HS and LS states alone, but can be qualitatively explained using a thermodynamics model for a mixture of HS and LS states across the spin crossover. We found that LS (Mg, Fe)O exhibits ∼6–8% lower solidus and liquidus temperature than its HS counterpart. Furthermore, our results show that iron preferentially partitions into melt within the spin crossover to generate iron-rich LS melt. Such iron-rich LS (Mg, Fe)O is ∼27(±5)% denser than materials expected for lowermost mantle and could potentially persist as residual melt in the lowermost mantle at the late stage of magma ocean crystallization. Modeled results indicate that the existence of the dense, iron-rich LS (Mg, Fe)O melt in the lowermost mantle could provide plausible explanations for characteristic seismological signatures of ultra-low velocity zones (ULVZs).
•We constructed temperature–composition phase diagram of (Mg, Fe)O up to ∼120 GPa.•We observed and modeled non-ideal melting of ferropericlase across the spin crossover.•The spin crossover in liquid (Mg, Fe)O was experimentally constrained the first time.•Significant iron partitions into the melt phase during the melting of ferropericlase.•The dense LS iron-rich (Mg, Fe)O melt in the lowermost mantle could explain ULVZs.
The intensity‐scan (I‐scan) technique to study the polarization‐dependent, nonlinear processes in exfoliated bulk ReS2 is utilized. The polarization‐dependent reflection and transmission of ReS2, ...from which the absorption coefficients are extracted using the transfer matrix method, are measured. Absorption coefficients under high laser peak power show a transition from saturable absorption (SA) to reverse saturable absorption when rotating the laser polarization with respect to the b‐axis. It is found that SA and excited‐state absorption (ESA) contribute to the nonlinear optical processes. Both the SA and ESA show strong dependence on the polarization angle, which is attributed to the anisotropic optical transition probability and electronic band structure in ReS2. The anisotropic nonlinear optical properties of ReS2 may find applications as saturable absorbers in lasers and optical modulators.
The polarization‐dependent nonlinear optical response of ReS2
is investigated through intensity‐scan technique. By controlling the laser polarization with respect to b‐axis, a saturable absorption to reverse saturable absorption crossover is observed due to the anisotropic optical transition probability and electronic band structure, which makes ReS2 a promising material in optical applications.
Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal ...conductivity at relevant pressure–temperature conditions of Earth's core–mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core–mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (∼1.1 W/m/K) is almost two times smaller than that of bridgmanite (∼2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core–mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.
•We report on optical absorption in iron-rich post-perovskite at 130 GPa and 2050 K.•Radiative conductivity of post-perovskite is ∼50% smaller than that of bridgmanite.•We construct a total thermal conductivity model of the D″ layer.•Global abundance of post-perovskite in D″ is up to ∼70%.
WN_{6} phase discovered at 126-165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group R3over ¯m which is consistent with the WN_{6} structure with armchairlike ...hexazine (N_{6}) rings, while strong A_{1g} Raman mode confirms its N─N single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent N_{6} rings, and repulsions between N_{6} rings that make WN_{6} ultrastiff and tough. The WN_{6} phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.
The synchrotron radiation technique of nuclear resonant inelastic X‐ray scattering (NRIXS), also known as nuclear resonance vibrational spectroscopy or nuclear inelastic scattering, provides a wealth ...of information on the vibrational properties of solids. It has found applications in studies of lattice dynamics and elasticity, superconductivity, heme biochemistry, seismology, isotope geochemistry and many other fields. It involves probing the vibrational modes of solids by using the nuclear resonance of Mössbauer isotopes such as 57Fe, 83Kr, 119Sn, 151Eu and 161Dy. After data reduction, it provides the partial phonon density of states of the Mössbauer isotope that is investigated, as well as many other derived quantities such as the mean force constant of the chemical bonds and the Debye velocity. The data reduction is, however, not straightforward and involves removal of the elastic peak, normalization and Fourier–Log transformation. Furthermore, some of the quantities derived are highly sensitive to details in the baseline correction. A software package and several novel procedures to streamline and hopefully improve the reduction of the NRIXS data generated at sector 3ID of the Advanced Photon Source have been developed. The graphical user interface software is named SciPhon and runs as a Mathematica package. It is easily portable to other platforms and can be easily adapted for reducing data generated at other beamlines. Several tests and comparisons are presented that demonstrate the usefulness of this software, whose results have already been used in several publications. Here, the SciPhon software is used to reduce Kr, Sn, Eu and Dy NRIXS data, and potential implications for interpreting natural isotopic variations in those systems are discussed.
SciPhon is a software for the reduction of nuclear resonant inelastic X‐ray scattering data. Tests and examples of applications to Fe, Kr, Sn, Eu and Dy data are presented.
Complex seismic, thermal, and chemical features have been reported in Earth's lowermost mantle. In particular, possible iron enrichments in the large low shear‐wave velocity provinces (LLSVPs) could ...influence thermal transport properties of the constituting minerals in this region, altering the lower mantle dynamics and heat flux across core‐mantle boundary (CMB). Thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of Earth's lower mantle. Importantly, the pressure‐induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity, but these effects remain largely unknown. Here we precisely measured the lattice thermal conductivity of Fe‐bearing bridgmanite to 120 GPa using optical pump‐probe spectroscopy. The conductivity of Fe‐bearing bridgmanite increases monotonically with pressure but drops significantly around 45 GPa due to pressure‐induced lattice distortion on iron sites. Our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought. The decrease in the thermal conductivity of bridgmanite in mid‐lower mantle and below would promote mantle flow against a potential viscosity barrier, facilitating slabs crossing over the 1000 km depth. Modeling of our results applied to LLSVPs shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease, which would enhance internal convective flow. Our CMB heat flux modeling indicates that while heat flux variations are dominated by thermal effects, variations in thermal conductivity also play a significant role. The CMB heat flux map we obtained is substantially different from those assumed so far, which may influence our understanding of the geodynamo.
Key Points
We combine time domain thermoreflectance and diamond cell to measure lattice thermal conductivity of Fe‐bearing bridgmanite to 120 GPa
Thermal conductivity of Fe‐bearing bridgmanite drops around 45 GPa due to pressure‐induced lattice distortion
Modeling of LLSVP thermal conductivity and CMB heat flux provides insights to thermo‐chemical structure and dynamics of lowermost mantle
Objectives
To investigate the structural changes of hippocampus and amygdala and their relationships with migraine frequency and prognosis.
Methods
Hippocampus and amygdala volumes were measured by ...3-T brain magnetic resonance imaging (MRI) in 31 controls and 122 migraine patients who were categorized into eight groups by headache frequency: group 1 (1–2 headache days/month), 2 (3–4), 3 (5–7), 4 (8–10), 5 (11–14), 6 (15–19), 7 (20–24), and 8 (25–30). Headache frequency was reassessed 2 years later and a frequency reduction ≥50% was regarded a good outcome.
Results
Hippocampus and amygdala volumes fluctuated in patient groups but did not differ from the controls. In migraine patients, the bilateral hippocampus volumes peaked in group 3. The volumes and headache frequencies correlated positively in groups 2–3 on bilateral sides (L: r = 0.44, p = 0.007; R: r = 0.35, p = 0.037), and negatively in groups 3–7 on the left side (5–24 days/month; L: r = −0.31, p = 0.004) and groups 3–8 on the right side (r = −0.31, p = 0.002). The left amygdala volume also peaked in group 3, and correlated with headache frequency in groups 1–3 (r = 0.34, p = 0.020) and groups 3–6 (r = −0.30, p = 0.012). The volumetric changes of the right amygdala with headache frequency did not reach statistical significance. At 2-year follow-up, the right hippocampus volume was positively associated with a good migraine outcome after adjustment of headache frequency (OR 4.72, p = 0.024).
Conclusions
Hippocampus and amygdala display a structural plasticity linked to both headache frequency and clinical outcome of migraine.
PDF
