Meteorites give us insight into the early development of the solar system. Using the SAPHiR instrument at the Research Neutron Source Heinz Maier-Leibnitz (FRM II) at the Technical University of Munich (TUM), a scientific team has for the first time simulated the formation of a class of stony-iron meteorites, so-called pallasites, on a purely experimental basis.
“Pallasites are the optically most beautiful and unusual meteorites,” says Dr. Nicolas Walte, the first author of the study, in an enthusiastic voice. They belong to the group of stony-iron meteorites and comprise green olivine crystals embedded in nickel and iron. Despite decades of research, their exact origins remained shrouded in mystery.
To solve this puzzle, Dr. Nicolas Walte, an instrument scientist at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, together with colleagues from the Bavarian Geoinstitute at the University of Bayreuth and the Royal Holloway University of London, investigated the pallasite formation process. In a first, they succeeded in experimentally reproducing the structures of all types of pallasites.
Deployment of the SAPHiR instrument
For its experiments, the team used the SAPHiR multi-anvil press which was set up under the lead of Prof. Hans Keppler of the Bavarian Geoinstitute at the MLZ and the similar MAVO press in Bayreuth. Although neutrons from the FRM II have not yet been fed into SAPHiR, experiments under high pressures and at high temperatures can already be performed.
“With a press force of 2400 tons, SAPHiR can exert a pressure of 15 gigapascals (GPa) on samples at over 2000 °C,” explains Walte. “That is double the pressures needed to convert graphite into diamond.” To simulate the collision of two celestial bodies, the research team required a pressure of merely 1 GPa at 1300 °C.
How are pallasites formed?
Until recently, pallasites were believed to form at the boundary between the metallic core and the rocky mantle of asteroids. According to an alternative scenario, pallasites form closer to the surface after the collision with another celestial body. During the impact molten iron from the core of the impactor mingles with the olivine-rich mantle of the parent body.
The experiments carried out have now confirmed this impact hypothesis. Another prerequisite for the formation of pallasites is that the iron core and rocky mantle of the asteroid have partially separated beforehand.
All this happened shortly after their formation about 4.5 billion years ago. During this phase, the asteroids heated up until the denser metallic components melted and sank to the center of the celestial bodies.
The key finding of the study is that both processes – the partial separation of core and mantle, and the subsequent impact of another celestial body – are required for pallasites to form.
Insights into the origins of the solar system
“Generally, meteorites are the oldest directly accessible constituents of our solar system. The age of the solar system and its early history are inferred primarily from the investigation of meteorites,” explains Walte.
“Like many asteroids, the Earth and moon are stratified into multiple layers, consisting of core, mantle and crust,” says Nicolas Walte. “In this way, complex worlds were created through the agglomeration of cosmic debris. In the case of the Earth, this ultimately laid the foundations for the emergence of life.”
The high-pressure experiments and the comparison with pallasites highlight significant processes that occurred in the early solar system. The team’s experiments provide new insights into the collision and material mixing of two celestial bodies and the subsequent rapid cooling down together. This will be investigated in more detail in future studies.
Reference: “Two-stage formation of pallasites and the evolution of their parent bodies revealed by deformation experiments” by Nicolas P. Walte, Giulio F. D. Solferino, Gregor J. Golabek, Danielle Silva Souza and Audrey Bouvier, 30 June 2020, Earth and Planetary Science Letters.
The research was funded by the German Federal Ministry of Education and Research (BMBF).
The relatively large grains of olivine and nickel-iron with obvious Widmanstätten pattern are strongly suggestive of a slow cooling, not the result of a transient event.
That is a good point and seems to be part of the problem that they want to solve. According to Wikipedia some pallasites show these slow cooling rate patterns of Ni-poor kamacite plates “between 700 and 450 ◦C” and the paper notes: “The wide variety of cooling rates in the temperature interval between ≈ 700–400◦C determined in pallasites was linked to different burial depths and taken as evidence against a core-mantle origin for pallasites (Yang et al., 2010).”
Their two-stage formation model is therefore set up so that it can cover this variety and so answer fundamental questions. [In the paper, besides being a possible witness to differentiation history of early solar system bodies: “1. Why is olivine the dominant and often singular silicate phase in pallasites? 2. Which process led to mixing between the olivine and the metal phase? 3. Why has the large density contrast between the olivines and the presumably molten metallo-sulphidic components not led to a subsequent gravitational separation? (The last they call “the pallasite problem”.)]
Specifically the paper notes on Widmanstätten patterns: “The intergrowth of kamacite and taenite lamellae in the form of Widmanstätten pattern or as plessite has been widely used to infer cooling rates for pallasites (Yang et al., 2010); however, in this study we do not consider these textural features as they form at temperatures well below the solidus of all phases and are therefore secondary.” Later in the paper they present their impact model, which seems to allow for slow cooling [Fih. 9]: “(d) An impact causes deformation of the mantle(top magnification)closely followed by intrusion of the impactor’s core melt (bottom magnification). (e) The impact rebound causes freezing of the metal melt followed by slow conductive cooling.” The later part of the cooling, taking place at the bottom of a deep crater next to the still metal melt mantle, may well take place on geological time scales as far as I know. [And they give answers of sorts to the 3 main questions, which are interesting re cooling rates – another two-stage process, of which I mentioned the last, slow part.]
“[Fih. 9]: “(d) An impact causes deformation of the mantle(top magnification)closely” = [Fig. 9]: “(d) An impact causes deformation of the mantle (top magnification) closely.
And I, confusingly, have their description of a two-stage cooling process, then claiming I had “mentioned the last, slow part”.
Sorry, I was waiting for the coffee to be sippable … I’m going to hit that brain refresh “button” now.
Everyone knows those olivine gems were put there by elves. But there is always going to be that one guy that says otherwise.
Anyone who owns meteorites is in contact with aliens. Meteorites are extra terrestrial. They are inorganic aliens (not organic). If I drop a meteorite on my big toe, I’ve been attacked by an alien!
Some meteorites came from Mars and Luna. Asteroid impacts can eject debris at higher than escape velocity into space. Earth is a gravity well and Martian rocks can be found in Northwest Africa (Sahara desert) and the Antarctic. I have pressed these meteorites to the bottom of my foot; therefore I claim I’ve walked on Mars and the Moon.