Dennis Harries

Fate of MgSiO3 melts at core–mantle boundary conditions

Significance A new technique has been developed to measure in situ the density of amorphous material composed of light elements under extreme conditions of pressure using the X-ray absorption method. At core–mantle boundary (CMB) pressure, the densities of MgSiO3 glass and melts are similar to the one of the crystalline bridgmanite, within uncertainty. Due to the affinity of iron oxide for silicate liquids, melting in the MgSiO3–FeSiO3 system will produce dense melts that could accumulate above the CMB, leading to the formation of a dense basal magma ocean in the early Earths mantle. One key for understanding the stratification in the deep mantle lies in the determination of the density and structure of matter at high pressures, as well as the density contrast between solid and liquid silicate phases. Indeed, the density contrast is the main control on the entrainment or settlement of matter and is of fundamental importance for understanding the past and present dynamic behavior of the deepest part of the Earth’s mantle. Here, we adapted the X-ray absorption method to the small dimensions of the diamond anvil cell, enabling density measurements of amorphous materials to unprecedented conditions of pressure. Our density data for MgSiO3 glass up to 127 GPa are considerably higher than those previously derived from Brillouin spectroscopy but validate recent ab initio molecular dynamics simulations. A fourth-order Birch–Murnaghan equation of state reproduces our experimental data over the entire pressure regime of the mantle. At the core–mantle boundary (CMB) pressure, the density of MgSiO3 glass is 5.48 ± 0.18 g/cm3, which is only 1.6% lower than that of MgSiO3 bridgmanite at 5.57 g/cm3, i.e., they are the same within the uncertainty. Taking into account the partitioning of iron into the melt, we conclude that melts are denser than the surrounding solid phases in the lowermost mantle and that melts will be trapped above the CMB.

Translation interface modulation in NC-pyrrhotites: Direct imaging by TEM and a model toward understanding partially disordered structural states

Abstract The crystallographic complexity of “hexagonal” or “intermediate” pyrrhotites (Fe1-xS with 0.125 > x > 0.080) is a long-standing and challenging problem. Integral (e.g., 5C) and non-integral NC type structures found within this group at ambient temperatures are characterized by sharp but complicated electron diffraction patterns, which were found to be interpretable in terms of a translation interface modulation (TIM) superstructure. Transmission electron microscopy (TEM) dark-field images obtained using superstructure reflections show dense arrangements of stripes, which can be interpreted as arrays of closely spaced anti-phase domain boundaries (APB). The displacement vector at the interface is R = 1/8[001] of a metrically hexagonal 4C superstructure cell and the involved translations are solely confined to the Fe sublattice. The vacancy arrangement of the APB-free monoclinic 4C-pyrrhotite serves as a base of the TIM superstructure and therefore NC structures can be regarded as two super-imposed ordering phenomena relating to the arrangements of individual vacancies and APBs, respectively. APBs are chemically non-conservative and govern the higher Fe/S ratios of intermediate NC-pyrrhotites. If oriented strictly parallel to (001) the APBs can be regarded as completely filled Fe double layers within the 4C stacking sequence. However, direct imaging of APBs shows waviness and variable disorder on mesoscopic scales, yielding essentially aperiodic structures. A high degree of self-organization among APBs has been observed within apparent diffusion profiles around exsolved troilite lamellae and along interfaces with 4C-pyrrhotite, where complicated eightfold node arrangements occur. Our TEM observations indicate that all NC-type pyrrhotites can be treated by the TIM approach and that the concepts of polytypism and polysomatism in pyrrhotite are not fully capable in representing the observed structural complexities.

Mineralogy and defect microstructure of an olivine-dominated Itokawa dust particle: evidence for shock metamorphism, collisional fragmentation, and LL chondrite origin

We report here detailed analytical scanning and transmission electron microscopic investigations on an olivine-dominated dust particle (RB-QD04-0042) from the surface of asteroid 25143 Itokawa. The dust particle was returned to Earth by the Hayabusa spacecraft and was made available in the context of the first announcement of opportunity for Hayabusa sample investigation. Multiple thin slices were prepared from the precious particle by means of focused ion beam thinning, providing a unique three-dimensional access to its interior. The 40 × 50 μm sized olivine particle contains a spherical diopside inclusion and an intimate intergrowth of troilite and tetrataenite. The compositions of olivine (Fo69Fa31) and diopside (En48Wo42Fs10), as well as the high Ni content of the sulfide-metal alloy, indicate a LL ordinary chondrite origin in accord with previous classifications. Although no impact crater exists at the surface of RB-QD04-0042, transmission electron microscopy revealed the presence of various shock defects in constituent minerals. These defects are planar fractures and [001] screw dislocations in olivine, multiple {101} deformation twins in tetrataenite and basal (0001) stacking faults in troilite. These diagnostic shock indicators occur only in a small zone on one concave side of the dust particle characterized by a high fracture density. These observations can be explained by a collisional event that spalled off material from the particles surface. Alternatively, the dust particle itself could be a spallation fragment of an impact into a larger regolith target. This suggests that Itokawa dust particles lacking visible microcraters on their surfaces might have still experienced shock metamorphism and were involved in collisional fragmentation that resulted in the formation of regolith.

The mineralogy and space weathering of a regolith grain from 25143 Itokawa and the possibility of annealed solar wind damage

We report the results of detailed mineralogical investigations by analytical scanning and transmission electron microscopy of particle RA-QD02-0115 recovered from the surface of asteroid 25143 Itokawa. We divided the 65 μm × 50 μm small particle into eight individual subsample slices via the focused ion beam method. The particle dominantly consists of olivine and contains inclusions of merrillite, tetrataenite/taenite, troilite, chromite, kamacite, and Cl-bearing apatite (in approx. decreasing order of frequency). The composition of olivine (fayalite 29.8 ± 1.1 mol% and molar Fe/Mn ratio of 57 ± 2) as well as the Ni-rich metal assemblage indicates an LL-type affinity in accord with previous classifications. The particle shows effects of solar wind irradiation on one of its principal faces. Olivine developed an approximately 34 nm wide rim composed of low-angle misoriented, nanometer-sized crystallites accompanied by a small amount of amorphous material. Exposed troilite developed a 4 to 8 nm wide polycrystalline rim with large-angle misorientations of the iron sulfide nanocrystallites. Merrilite shows marginally discernable surface damage but was too unstable under the electron beam for a detailed study. Cl-bearing apatite was found fully crystalline with no discernable rim structure. We discuss the unusual polycrystalline nature of the olivine rim in terms of possible annealing and recrystallization effects, which may have occurred during periods of time when Itokawa’s surface temperature may have been warmer due to closer perihelion distances. Model calculations show that the dynamical orbital evolution of near-Earth asteroids could lead to complex space weathering processes, arising from the competing interplay between irradiation-induced damaging and thermally driven annealing.

Reproducing space weathering of olivine by using high-energy femtosecond laser pulses

Atmospheric-free bodies of the solar system are undergoing several processes that alter their original spectral characteristics. The whole of these processes is the so-called space weathering. The surface of such bodies is exposed to the solar wind irradiation and to the ongoing bombardment of micrometeoroids yielding material modifications at the micro- and nanometer scale. In order to understand these processes and clarify the influence on spectral reflectance and absorption, numerous experimental approaches using ion and laser irradiation have been presented so far. However, up to this date, basic damaging mechanisms are still unresolved or cannot be completely reproduced. In this work, we present the application of ultra-short laser pulses as a tool to reproduce space weathering, with focus on micrometeoroid impacts. In our experiments, slices of single-crystal olivine were irradiated under vacuum condition using 100 fs single-shot laser pulses. In order to perform spectral measurements, the laser-damaged regions were distributed over the sample surface within a grid geometry. After laser processing, a comprehensive study was performed by using spectroscopic measurements in the NUV-vis-NIR range, white light interferometry, SEM and TEM analysis. The cross-sections of the laser-generated craters reveal different layers including from the top to the bottom: an amorphous layer, two polycrystalline layers with different textures, and a defect-rich olivine substrate. Moreover, iron nanoparticles occur within the lower part of the amorphous layer and the polycrystalline layer. We can reproduce microcraters whose morphology, microstructure, and distribution of iron nanoparticles are similar to those found in the soil samples of the Moon or of the asteroid 25143 Itokawa.