Erg Chech 002 (EC 002) is an ungrouped achondrite meteorite discovered in the Erg Chech region of the Sahara Desert in Algeria in May 2020. Classified as an andesite, this meteorite is believed to originate from a chondritic protoplanet that is over 4.566 billion years old, making it one of the oldest known pieces of volcanic rock in the solar system – approximately 20 million years older than Earth itself. It is characterized by its coarse-grained texture, with a unique composition including large greenish xenocrysts of pyroxene and olivine. This meteorite provides invaluable insights into the early solar system’s igneous processes and the formation of planetesimal crusts.
A typical slice of Erg Chech 002 looks like it is shown in the images below.
However, there was a single stone (that was cut out in a few thin slices) that looked quite different. The slices display a spinifex-like texture. You may read about spinifex texture here https://www.alexstrekeisen.it/english/vulc/spinifex.php. Erg Chech 002 is Andesite, which means that it has a much lower magnesium content that is the necessary chemical conditions for the formation of large, skeletal crystals of olivine or pyroxene (the spinifex texture).
I hypothesize that the presence of large olivine xenocrysts, which according to Barrat et al. (https://www.pnas.org/doi/10.1073/pnas.2026129118) “are more likely derived from the crystallization of associated magma(s) more magnesian than Erg Chech 002“, and their interaction with the Erg Chech 002 magma is responsible for at least some parts of the spinifex-like texture in Steve’s and Bradley’s slices.
What are xenocrysts? In the context of Erg Chech 002 (EC 002), xenocrysts are mineral crystals that were not formed from the melt that constitutes the meteorite itself but were instead incorporated from other, older rocks or magma sources. In EC 002, these xenocrysts include minerals like olivine, which are different in composition from the surrounding matrix of the meteorite. Their presence suggests that the rock underwent complex geological processes, including the mixing of different magma sources during its formation. In their article (https://iopscience.iop.org/article/10.3847/1538-4357/ad2ea7), Ziliang Jin et al. write:
Our research reveals that these xenocrysts were early formed crystals, loosely accumulated or scattered in the short-period magma ocean on the parent body.
Ziliang Jin et al. concluded that in Erg Chech 002 xenocrysts that supposedly originated from more primitive melts are set in an evolved andesitic (Erg Chech 002) groundmass. Let us try to visualize the magma ocean, in which new magma is incorporating green crystals from older, more primitive melt.
These xenocrysts interact with magma in different ways, creating reaction rims, reaction coronas and zoning. See examples of the reaction rims and reaction coronas photographed from thin sections of Erg Chech 002 below.
And sometimes reaction coronas and crystals overgrow look like in the image displayed below. The corona and overgrow around the white xenocryst look like a mini spinifex texture.
And what happens if a few xenocrysts are located in a close proximity to each other?
In such cases, xenocrysts interacting with magma can change the texture of the stone completely as it is shown in the image below.
And close up.
Xenocrysts can affect the texture of the host rock in several ways. We have already talked about reaction rims, reaction coronas and overgrow. Let us discuss another two possibilities. In magma, these foreign crystals can act as nucleation sites where new crystals begin to grow. This occurs because the surface of a xenocryst provides a lower energy site for new mineral phases to nucleate compared to forming a new crystal from scratch in the melt. Heterogeneous Nucleation process is known as heterogeneous nucleation where the presence of a pre-existing surface (the xenocryst) facilitates the crystallization of minerals from the melt. It’s energetically more favorable than homogeneous nucleation, where new crystals form without any pre-existing surfaces. A partial melting of xenocrysts also can significantly change the texture. You might observe compositional zoning or textural changes in the xenocrysts where the melting occurred, indicating where and how the crystal reacted with the melting. This could show up as changes in crystal structure or chemistry at the edge of the xenocryst. Partial melting can lead to the entrapment of melt inclusions within the xenocryst. These inclusions can provide direct evidence of the composition of the melt at the time of partial melting. The distribution of trace elements might show anomalies or patterns that suggest melting and recrystallization processes. Melting could cause diffusion of elements into or out of the crystal, changing its trace element signature. In fact, Jean-Alix Barrat et al. (https://www.pnas.org/doi/10.1073/pnas.2026129118) noticed that “pyroxenes surrounding the crystal are more magnesian than those in the groundmass”, which I believe proves that a partial melting of xenocrysts did occur. Now, if we are to look at Steven’s and Bradley’s images one more time, we can clearly see that at least some of the unusual texture of the slice is associated with xenocrysts. Some xenocrysts could have melted completely but not before they changed the texture around them.
Please also notice that in many instances the straight thin lines radiating from xenocrysts in Steven’s and Bradley’s images look similar to the straight thin lines radiating from xenocrysts in the thin section below.
The thin section was made out of the middle portion of the slice, presented below.
I propose that an unusual spinifex-like texture in Steven’s and Bradley’s material resulted from xenocrysts interacting with the magma, maybe partial melting and recrystallization.
However, the spinifex-like texture mystery in Erg Chech 002 does not end with Steven’s and Bradley’s slices. A few little individuals were also reported to have a similar texture. A typical Erg Chech 002 individual looks as in the images shown below.
Then what happened to the one shown below?
And a few microscopic images of the same meteorite.
Maybe the texture of this little individual (1.22 grams and about 10 mm across) is a result of impact event in meteoritic parent-body. In 1976 R.V. Fodor, Klaus Keil published an article about a komatiite-like lithic fragment with spinifex texture which is similar to that of terrestrial komatiites, in the Eva (ordinary chondrite) meteorite. The authors concluded that this texture is characterized by skeletal olivine crystals embedded in a glassy matrix, indicating that the fragment underwent rapid cooling and crystallization from a supercooled melt. Although the composition of the lithic fragment showed significant differences from the host meteorite, a genetic relationship between the two was noticed. The authors propose that the lithic fragment is likely the result of impact melting of the host chondrite material. This process involved the melting of olivine and orthopyroxene-rich material, followed by rapid cooling and crystallization, which produced the observed spinifex texture.
The images of Eva meteorite, described in Fodor’s article, look very similar to the one, photographed by Mirko Graul below.
In fact, it is a well-known fact that this kind of texture can be found in impact melts. For example, see figures 6.13, 6.14 and 6.17 from this article https://www.lpi.usra.edu/publications/books/CB-954/chapter6.pdf The author does not call this “a spinifex texture”. Instead, the description “quench texture” is used.
Maybe my little individual is impact melt that survived the journey to the earth on its own. Jean-Alix Barrat et al. (https://www.pnas.org/doi/10.1073/pnas.2026129118) noted that the presence of silica polymorphs that in Erg Chech 002 significantly affected the cooling rate, making it much faster. They concluded that “the most likely explanation for this change in cooling rate is an impact that would have excavated, or more likely ejected, the rock from its parent body”.
As long as we have already mentioned quench texture let us talk about one more example that is found in meteorites. In their article, Fodor and Keil also mention an interesting observation in regard to the chondrules. Many of them have a quench texture too (or as Fodor and Keil call them “spinifex” texture), which as Fodor and Keil state is not surprising, considering that chondrules, formed by crystallization from supercooled melts. See the images of chondrules below.
I have shown that the spinifex-like texture observed in Steven’s and Bradley’s slices is likely connected to the presence of xenocrysts in some manner. Nevertheless, I am unable to account for the rarity of this phenomenon, especially considering that xenocrysts are relatively abundant in Erg Chech 002.
Acknowledgements
I am grateful to Steven Dixey, Bradley Carter and Mirko Graul for allowing using their images in this article.
