Natural (Mg,Fe)SiO3-ilmenite and -perovskite in the Tenham meteorite

Unique evidence of fluid alteration in the Kakowa (L6) ordinary chondrite

Meteorites preserve evidence of processes on their parent bodies, including alteration, metamorphism, and shock events. Here we show that the Kakowa (L6) ordinary chondrite (OC) preserves both shock-melt veins and pockets of detrital grains from a brecciated and altered object, including corundum, albite, silica, fayalite, forsterite, and margarite in a Pb- and Fe-rich matrix. Preservation of the observed mineralogy and texture requires a sequence of at least two impacts: first, a high-velocity collision formed the shock melt veins containing the high-pressure minerals ringwoodite, wadsleyite, majorite, and albitic jadeite; later, a low-velocity impact formed fractures and filled them with the detrital material. Oxygen and Pb isotope ratios suggest an OC origin for these detrital minerals. Although fluid alteration is common in carbonaceous chondrites, the discovery of margarite with an OC oxygen isotopic signature is novel. Kakowa extends both the impact and alteration history of L6 ordinary chondrites in general.

Formation, preservation and extinction of high-pressure minerals in meteorites: temperature effects in shock metamorphism and shock classification

The goal of classifying shock metamorphic features in meteorites is to estimate the corresponding shock pressure conditions. However, the temperature variability of shock metamorphism is equally important and can result in a diverse and heterogeneous set of shock features in samples with a common overall shock pressure. In particular, high-pressure (HP) minerals, which were previously used as a solid indicator of high shock pressure in meteorites, require complex pressure-temperature-time (P-T-t) histories to form and survive. First, parts of the sample must be heated to melting temperatures, at high pressure, to enable rapid formation of HP minerals before pressure release. Second, the HP minerals must be rapidly cooled to below a critical temperature, before the pressure returns to ambient conditions, to avoid retrograde transformation to their low-pressure polymorphs. These two constraints require the sample to contain large temperature heterogeneities, e.g. melt veins in a cooler groundmass, during shock. In this study, we calculated shock temperatures and possible P-T paths of chondritic and differentiated mafic-ultramafic rocks for various shock pressures. These P-T conditions and paths, combined with observations from shocked meteorites, are used to constrain shock conditions and P-T-t histories of HP-mineral bearing samples. The need for rapid thermal quench of HP phases requires a relatively low bulk-shock temperature and therefore moderate shock pressures below ~ 30 GPa, which matches the stabilities of these HP minerals. The low-temperature moderate-pressure host rock generally shows moderate shock-deformation features consistent with S4 and, less commonly, S5 shock stages. Shock pressures in excess of 50 GPa in meteorites result in melt breccias with high overall post-shock temperatures that anneal out HP-mineral signatures. The presence of ringwoodite, which is commonly considered an indicator of the S6 shock stage, is inconsistent with pressures in excess of 30 GPa and does not represent shock conditions different from S4 shock conditions. Indeed, ringwoodite and coexisting HP minerals should be considered as robust evidence for moderate shock pressures (S4) rather than extreme shock (S6) near whole-rock melting.

Discovery of davemaoite, CaSiO3-perovskite, as a mineral from the lower mantle

[Figure: see text].

Natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite

Bridgmanite, the most abundant mineral of the Earth's lower mantle, has been reported in only a few shocked chondritic meteorites; however, the compositions of these instances differ from that expected in the terrestrial bridgmanite. Here, we report the first natural occurrence of Fe-bearing aluminous bridgmanite in shock-induced melt veins within the Katol L6 chondrite with a composition that closely matches those synthesized in high-pressure and temperature experiments over the last three decades. The Katol bridgmanite coexists with majorite and metal-sulfide intergrowths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe³⁺/ΣFe ratio (0.69 ± 0.08) than coexisting majorite (0.37 ± 0.10), which agrees with experimental studies. The Katol bridgmanite is arguably the closest natural analog for the bridgmanite composition expected to be present in the Earth's lower mantle. Textural observations and comparison with laboratory experiments suggest that the Katol bridgmanite formed at pressures of ∼23 to 25 gigapascals directly from the chondritic melt generated by the shock event. Thus, the Katol L6 sample may also serve as a unique analog for crystallization of bridgmanite during the final stages of magma ocean crystallization during Earth's formation.

Crystallographic orientation of ilmenite inclusions in amphibole – an electron backscatter diffraction study

Orientated ilmenite inclusions have been discovered in amphibole of hornblendite from the Zhujiapu area, Dabie ultra-high-pressure (UHP) metamorphic terrane, China. In order to characterize the crystallographic orientation relationships between ilmenite inclusions and amphibole host and reconstruct the mechanism of their formation, we present an electron backscatter diffraction (EBSD) analysis combined with energy dispersive spectroscopy (EDS) analysis and electron microprobe analysis (EPMA) for ilmenite inclusions and amphibole host. The inclusions can be subdivided into four groups: (1) 60.2% of ilmenites have the crystallographic orientation {0001}Ilm // {100}Amp, (101̅0)Ilm // {010}Amp, [112̅0]Ilm // <001> Amp and [112̅0]Ilm // <012 > Amp. (2) 16.5% of ilmenites have <0001> Ilm // <001> Amp, (101̅0)Ilm // {010}Amp, (112̅0)Ilm // {100}Amp and [3̅031]Ilm // <012> Amp. (3) 13.8% of ilmenites have <0001> Ilm // <012> Amp, (112̅0)Ilm // {100}Amp and [3̅031]Ilm // <001> Amp. (4) 9.5% of ilmenites have <0001> Ilm // [1̅12]Amp, (101̅0)Ilm // {201}Amp, [112̅0]Ilm // [1̅12]Amp and [ 11 21 ¯ ] I l m ${[11\overline {21} ]_{Ilm}}$ // <010> Amp. By comparing the lattice relationship between ilmenite inclusions and amphibole hosts, it is shown that the frequency of the ilmenite inclusions in different groups is related to the lattice coherency and oxygen packing. Group-1 of the ilmenite inclusions was most likely be formed via a solid-state exsolution process by cooling of the hornblendite after the intrusion was emplaced. The other three groups of ilmenite inclusions were probably formed via reduction reaction in an open system. The formation temperature of the ilmenite inclusions is estimated by using the TiO2 solubility geothermeter in amphibole. The minimum formation temperature of the ilmenite inclusions is about 1025 °C, and the maximum formation temperature of the ilmenite inclusions is about 1126 °C.

Evidence for the charge disproportionation of iron in extraterrestrial bridgmanite

Bridgmanite, MgSiO₃ with perovskite structure, is considered the most abundant mineral on Earth. On the lower mantle, it contains Fe and Al that strongly influence its behavior. Experimentalists have debated whether iron may exist in a mixed valence state, coexistence of Fe²⁺ and Fe³⁺ in bridgmanite, through charge disproportionation. Here, we report the discovery of Fe-rich aluminous bridgmanite coexisting with metallic iron in a shock vein of the Suizhou meteorite. This is the first direct evidence in nature of the Fe disproportionation reaction, which so far has only been observed in some high-pressure experiments. Furthermore, our discovery supports the idea that the disproportionation reaction would have played a key role in redox processes and the evolution of Earth.

High pressure minerals in the Château-Renard (L6) ordinary chondrite: implications for collisions on its parent body

We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyrope_ss, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteorite and the L-chondrites in general.

A new high-pressure form of Mg2SiO4 highlighting diffusionless phase transitions of olivine

High-pressure polymorphism of olivine (α-phase of Mg₂SiO₄) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg₂SiO₄). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg₂SiO₄ polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.

Discovery of the Fe-analogue of akimotoite in the shocked Suizhou L6 chondrite

We report the first natural occurrence of the Fe-analogue of akimotoite, ilmenite-structured MgSiO₃, a missing phase among the predicted high-pressure polymorphs of Fe-pyroxene, with the composition (Fe²⁺_0.48Mg_0.37Ca_0.04Na_0.04Mn²⁺_0.03Al_0.03Cr³⁺_0.01)_Σ=1.00Si_1.00O₃. The new mineral was approved by the International Mineralogical Association (IMA 2016-085) and named hemleyite in honour of Russell J. Hemley. It was discovered in an unmelted portion of the heavily shocked L6 Suizhou chondrite closely associated to olivine, clinoenstatite and Fe-bearing pyroxene with a composition nearly identical to that of hemleyite. We also report the first single-crystal X-ray diffraction study of a Si-bearing, ilmenite-structured phase. The fact that hemleyite formed in a meteorite exposed to high pressures (<20 GPa) and temperatures (<2000 °C) during impact-induced shocks indicates that it could play a crucial role at the bottom of the Earth’s mantle transition zone and within the uppermost lower mantle.

Discovery of natural MgSiO3 tetragonal garnet in a shocked chondritic meteorite

MgSiO3 tetragonal garnet, which is the last of the missing phases of experimentally predicted high-pressure polymorphs of pyroxene, has been discovered in a shocked meteorite. The garnet is formed from low-Ca pyroxene in the host rock through a solid-state transformation at 17 to 20 GPa and 1900° to 2000°C. On the basis of the degree of cation ordering in its crystal structure, which can be deduced from electron diffraction intensities, the cooling rate of the shock-induced melt veins from ~2000°C was estimated to be higher than 10(3)°C/s. This cooling rate sets the upper bound for the shock-temperature increase in the bulk meteorite at ~900°C.

High-quality EuO thin films the easy way via topotactic transformation

Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, but nonetheless powerful means of creating such films is via topotactic transformation, in which a chemical reaction transforms a single crystal of one phase into a single crystal of a different phase, which inherits its orientation from the original crystal. Topotactic reactions may be applied to epitactic films to substitute, add or remove ions to yield epitactic films of different phases. Here we exploit a topotactic reduction reaction to provide a non-ultra-high vacuum (UHV) means of growing highly oriented single crystalline thin films of the easily over-oxidized half-metallic semiconductor europium monoxide (EuO) with a perfection rivalling that of the best films of the same material grown by molecular-beam epitaxy or UHV pulsed-laser deposition. As the technique only requires high-vacuum deposition equipment, it has the potential to drastically improve the accessibility of high-quality single crystalline films of EuO as well as other difficult-to-synthesize compounds.

In the absence of matching substrates, the growth of oxide thin films can be challenging. Here, the authors demonstrate the growth of EuO thin films via a topotactic reaction, where a chemical reaction transforms a single crystal of one phase into that of another.

Earth science. Bridgmanite--named at last

The most abundant mineral in Earth's interior gets a name [Also see Report by Tschauner et al. ]

Mineral phases of the Earth´s mantle

Our knowledge of the structure of the Earth´s interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth´s mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth´s mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth´s mantle.

Ultrafast growth of wadsleyite in shock-produced melts and its implications for early solar system impact processes

We observed micrometer-sized grains of wadsleyite, a high-pressure phase of (Mg,Fe)(2)SiO(4,) in the recovery products of a shock experiment. We infer these grains crystallized from shock-generated melt over a time interval of <1 micros, the maximum time over which our experiment reached and sustained pressure sufficient to stabilize this phase. This rapid crystal growth rate (approximately 1 m/s) suggests that, contrary to the conclusions of previous studies of the occurrence of high-pressure phases in shock-melt veins in strongly shocked meteorites, the growth of high-pressure phases from the melt during shock events is not diffusion-controlled. Another process, such as microturbulent transport, must be active in the crystal growth process. This result implies that the times necessary to crystallize the high-pressure phases in shocked meteorites may correspond to shock pressure durations achieved on impacts between objects 1-5 m in diameter and not, as previously inferred, approximately 1-5 km in diameter. These results may also provide another pathway for syntheses, via shock recovery, of some high-value, high-pressure phases.

Structure of vapour deposited adenine on a nanostructured perovskite surface studied by STM

The structures of vapour deposited layers of adenine on a nanostructured SrTiO3(001) surface have been investigated by scanning tunneling microscopy (STM). The STM images reveal details of self-organization of adenine monolayers in which adsorption is controlled both by substrate nanostructure and by intermolecular H-bonding of adenine molecules. Detailed examination of STM images suggests that at least three different ordering structures are possible and two distinct orientations may exist with opposite chirality.

Timescales of shock processes in chondritic and martian meteorites

The accretion of the terrestrial planets from asteroid collisions and the delivery to the Earth of martian and lunar meteorites has been modelled extensively. Meteorites that have experienced shock waves from such collisions can potentially be used to reveal the accretion process at different stages of evolution within the Solar System. Here we have determined the peak pressure experienced and the duration of impact in a chondrite and a martian meteorite, and have combined the data with impact scaling laws to infer the sizes of the impactors and the associated craters on the meteorite parent bodies. The duration of shock events is inferred from trace element distributions between coexisting high-pressure minerals in the shear melt veins of the meteorites. The shock duration and the associated sizes of the impactor are found to be much greater in the chondrite (approximately 1 s and 5 km, respectively) than in the martian meteorite (approximately 10 ms and 100 m). The latter result compares well with numerical modelling studies of cratering on Mars, and we suggest that martian meteorites with similar, recent ejection ages (10(5) to 10(7) years ago) may have originated from the same few square kilometres on Mars.

Natural NaAlSi(3)O(8)-hollandite in the shocked sixiangkou meteorite

The hollandite high-pressure polymorph of plagioclase has been identified in shock-induced melt veins of the Sixiangkou L6 chondrite. It is intimately intergrown with feldspathic glass within grains previously thought to be "maskelynite." The crystallographic nature of the mineral was established by laser micro-Raman spectroscopy and x-ray diffraction. The mineral is tetragonal with the unit cell parameters a = 9.263 +/- 0.003 angstroms and c = 2.706 +/- 0.003 angstroms. Its occurrence with the liquidus pair majorite-pyrope solid solution plus magnesiowustite sets constraints on the peak pressures that prevailed in the shock-induced melt veins. The absence of a calcium ferrite-structured phase sets an upper bound for the crystallization of the hollandite polymorph near 23 gigapascals.