ATACAMAITES

Written by Aubrey Whymark, July 2021
Atacamaites are small tektite-like bodies. Five specimens of Atacamaites were discovered in 2012 by M. Warner and M. Warner (Devouard et al., 2014) or first recognized by M. Warner in 2011 (Gattacceca et al., 2021). In 2013 they managed to recover 3,000 specimens in a 20 km2 area (Devouard et al., 2014). Subsequent fieldtrips by other parties have yielded many more specimens: ∼23,000 samples from over 100 discrete locations over a surface of ∼650 km2 (Gattacceca et al., 2021). Utilizing the fission-track dating method, Atacamaites were dated to 7.83 ±0.26 Ma (Gattacceca et al., 2021). It would be interesting to confirm this date with Ar-Ar dating typically used to date impact glasses. Atacamaites appear to be derived from an impact involving an iron meteorite (Koeberl, Crósta & Schulz, 2019) and further restricted by Gattacceca et al. (2021) to a likely a IIAB Group Iron based on estimated 5.4 wt.% Ni content and the 12.0 Ni/Co ratio.

Tektites or Tektoids?

I would consider Atacamaites to be a form of proximal tektite. In smaller impacts the proximal forms are often smaller (likely because of lower altitude of formation and possibly higher initial velocities); they have higher impactor contamination; they are often ligamented forms: rather imperfect surface tension-controlled bodies. Had they not cooled and 'frozen' in non-equilibrium forms the ligaments would have formed droplets.

Atacamaite Key Data

Age: 7.83 ±0.26 Ma (Gattacceca et al., 2021).

Impactor: Iron Meteorite (IIAB) (Gattacceca et al., 2021).

Proximal tektites.

Antofagasta area, Atacama, Chile

Atacamaites are tektite-like bodies. Gattacceca et al. (2021) proposed a new term ‘tektoids’ for these bodies. It is a term that I don’t think will catch on and is probably unnecessary. It’s a tricky area because the observations in this article are quite correct and there are differences between proximal ejecta in large and small impacts. For some time I also felt this kind of body should be classified separately, but I think a sub-category of tektites might be better, so I will explain this before moving on to compare Atacamaites with other tektites / impact glasses.

I would define a tektite as:
  • Glass: Holohyaline (wholly glassy and macroscopically homogeneous). To form a glass a high percentage of network former (primarily SiO2 is present).
  • A droplet with surfaces controlled by surface tensions.
  • Ballistically ejected.

So do Atacamaites meet these criteria? Well, basically yes. Most Atacamaites are a holohyaline glass. Around 10% of Atacamaites reportedly contain crystallites which appear to be due to the presence of meteoritic contamination (Gattacceca et al., 2021). These specimens might better be termed Krystites. Atacamaites are droplets, but many might be termed as having ligaments. They have not quite got hot enough for long enough to form true droplets and are thus transitional between melted blocks and droplets at the point they cooled. Finally, Atacamaites are clearly ballistically ejected.

To illustrate this, see the figure below and Google ‘fluid jet break-up’. In an impact, molten material travels up through the atmosphere in an ejecta curtain. At a certain altitude, dependent on the forces involved, the ejecta curtain will form a crown with surface tension-controlled droplets extending from ligaments. Eventually the ligaments collapse to droplets so long as the ligaments have not solidified in the interim. Droplets will go on to be deformed through atmospheric interaction and cascade to smaller droplets before the (equilibrium of non-equilibrium) forms are locked in by the cooling material. The impact is a finite event, and the ejecta curtain is accelerated for a finite time. Eventually the ejecta curtain breaks-up, holes and ligaments form and the curtain collapses.

At distance in large impacts spherical droplets (modified by atmospheric interaction) are formed. As the energies dissipate in large impacts then closer to the source the lower temperature / lower shock yields teardrop morphologies from a viscous melt, then closer still (on average and overlapping) we find ligament forms and still closer layered forms, moving towards partial melts. In a smaller scaled down impact like Darwin Crater the droplets only just form (representing the more distal and highest temperature melts) but then we find these ligament forms and then still closer to the impact site we see blocky forms. There is a difference between the large and small impacts in that proximal forms in large impacts are last formed with low impactor contamination whereas proximal forms in a smaller impact may be the first formed bodies with the highest impactor contamination (often resulting in crystallites). Proximal forms in small impacts are usually small bodies under 1 or 2 centimeters whereas in larger impacts they are larger. This likely reflects the atmospheric density in which the bodies form: in large impacts many proximal tektites form at greater altitude which would result in less disruption and break-up / cascading.
  
ABOVE: An ejecta curtain showing development over time (left to right). Droplets are found at distance. Ligaments and blocky melts form closer to the source.
LEFT: Typical Atacamaites from Koeberl (2019)
So, in essence, I would consider Atacamaites as Tektites but then subdivide the Tektite groups into Proximal (plastically deformed, e.g., Indochinites), Medial (slightly plastically deformed, heated during re-entry, and spalled, e.g., Philippinites), and Distal (very slightly plastically deformed heated during re-entry to a degree they ablate, then spalled, e.g., Australites). I would then subdivide the Proximal forms into Layered, Ligament, and Droplet forms. Both the Layered and Ligament forms are on the margins of the criteria for tektites.

Atacamaites clearly represent a very proximal impact glass with great similarities to:
  • Irghizites derived from the 14 km diameter Zhamanshin Crater and located in the crater depression (Florenskij, 1976; Bouška et al., 1981).
  • Lonar Crater Glass derived from the 1.88 km Lonar Crater and found within several hundred meters of the crater rim (Dutta et al, 2019),
  • Darwin Glass derived from Darwin Crater and found within 21.5 km of the crater (Fudali & Ford, 1979),
  • Waber Glass from the Waber Craters - maximum 116 m diameter and found within 0.5 km.
  • Aouelloul Glass from the 390 m diameter Aouelloul Crater and found within 1 km of the crater (Lehrman, 2015). This glass is blockier in general.

So, what can we infer about the Atacamaite crater? It is clearly close to the strewnfield, most likely within tens of kilometers – it is even possible that the crater is within the strewnfield. The crater diameter will range from hundreds of meters to 14 km in diameter (perhaps somewhere in the middle). I would personally guess in the range of a couple to a few kilometers. It was formed by an iron meteorite (Koeberl, Crósta & Schulz, 2019; Gattacceca et al., 2021). It is interpreted as 7.83 ±0.26 Ma (Gattacceca et al., 2021). At this age any crater may be heavily eroded, buried with alluvial sediments, or buried beneath lava flows.

When I first heard about Atacamaites I immediately thought of the 350 x 370 m diameter Monturaqui Crater, which is located 185 km to the northwest. This crater is elongated in a northwest-southeast direction and the impactor probably came from the northwest (Kaniansky et al., 2015). Monturaqui Crater is calculated to be 0.663 ± 0.09 Ma (Ukstons Peate, 2010). It is interpreted to have been produced from an IAB Iron Meteorite (Kaniansky et al., 2015; Schmieder & Kring, 2020). Atacamaites appear to be incompatible with a Monturaqui source as the crater is probably too small and too far away, the age appears to be a mismatch, and whilst both suggest an iron meteorite impactor there is a mismatch in type. I would love to see some Ar-Ar dating though on Atacamaites and Monturaqui Crater glass to thoroughly discount any link.
ABOVE: The distribution of Atacamamites (Gattacceca et al., 2021) in relation to Monturaqui Crater which is the closest known confirmed impact crater, but an unlikely source. Google Earth Imagery.

References

Blog Atacama. 2017. https://labodessavoirs.fr/blog-atacama/

Bouška V., Povondra P., Florenskij P. V., Řanda Z. 1981. Irghizites and zhamanshinites: Zhamanshin crater, USSR. Meteoritics. 16 (2): 171-184.

Devouard B., Rochette P., Gattacceca J., Barrat J.-A., Moustard F., Valenzuela E. M., Alard O., Balestrieri M. L., Bigazzi G., Dos Santos E., Gounelle M., Jambon A., Laridhi-Ouazza N., Shuster D. L., Warner M., Warner M. 2014. A new Tektite Strewnfield in Atacama, Chile. Annual Meeting of the Meteoritical Society. 77th: Abstract #5394. Full article available free at http://www.hou.usra.edu/meetings/metsoc2014/pdf/5394.pdf

Dos Santos E., Scorzelli R. B., Rochette P., Devouard B., Gattacceca J., Moustard F., Cournède C. 2015. A New Strewnfield of Splash-Form Impact Glasses in Atacama, Chile: A Mössbauer Study. 78th Annual Meeting of the Meteoritical Society. 78th: Abstract #5074. Full article available free at https://www.hou.usra.edu/meetings/metsoc2015/pdf/5074.pdf

Dutta A., Raychaudhuri D., Pachpor S. V., Bhattacharya A., Bhattacharya A. 2019. Morphometric and Petrochemical Characterization of Impact Melt Spherules from Lonar Crater, Maharashtra, India. Abstracts of the Lunar and Planetary Science Conference. 50th: Abstract #1038. Full article available free at https://www.hou.usra.edu/meetings/lpsc2019/pdf/1038.pdf

Florenskij P. V. 1976b. The first tektite deposits in a meteoritic crater (Zhamanshin North Aral Region, USSR). Abstracts of Papers Presented to the Symposium on Planetary Cratering Mechanics. Lunar and Planetary Institute Contribution 259: 33-35. (Abstract). Full article available free at http://adsabs.harvard.edu/abs/1976LPICo.259...33F

Fudali R. F., Ford, R. J. 1979. Darwin glass and Darwin crater: a progress report. Meteoritics. 14: 283-296. Full article available free at http://adsabs.harvard.edu/abs/1979Metic..14..283F

Gattaccecaa J., Devouard B., Barrat J.-A., Rochette P., Balestrieri M. L., Bigazzi G., Ménard G., Moustard F., Dos Santos E., Scorzelli R., Valenzuela M., Gounelle M., Debaille V., Beck P., Bonal L., Reynard B., Warner M. 2021. A 650 km2 Miocene strewnfield of splash-form impact glasses in the Atacama Desert, Chile. Earth and Planetary Science Letters. 569: 117049.
Howard, K. T. 2011. Volatile enhanced dispersal of high velocity impact melts and the origin of tektites. Proceedings of the Geologists' Association. 122 (3): 363-382.

Kaniansky, Stanislav; Molnár, Kristian. 2015. A new analysis of Monturaqui Meteorites (PDF). Proceedings of the IMC. Mistelbach.

Koeberl C., Crósta A. P., Schulz T. 2019. Geochemical Investigation of the Atacamaites, a New Impact Glass Occurrence in South America. Abstracts of the Lunar and Planetary Science Conference. 50th: Abstract #1255. Full article available free at https://www.hou.usra.edu/meetings/lpsc2019/pdf/1255.pdf

Lehrman N. 2015a. Aouelloul Glass, Adrar, Mauritania. Meteorite Times (Web-based magazine). Norm’s Tektite Teasers. 14 (1) (January). Full article available free at https://www.meteorite-times.com/article-archives/

Lehrman N. 2015b. Atacamaites, Central Atacama Desert, Chile. Meteorite Times (Web-based magazine). Norm’s Tektite Teasers. 14 (2) (March). Full article available free at https://www.meteorite-times.com/article-archives/

Osae S., Misra S., Koeberl C., Sengupta D., Ghosh S. 2005. Target rocks, impact glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry. Meteoritics & Planetary Science. 40: 1473-1492. Full article available free at http://www.univie.ac.at/geochemistry/koeberl/publication_list/270-Lonar-Osae%20et%20al-MAPS2005.pdf

Schmieder M., Kring D. A. 2020. Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits. Astrobiology. 20(1): 91-141.

Shoemaker E. M., Wynn J. C. 1997. Geology of the Waber Meteorite Craters, Saudi Arabia. Abstracts of the Lunar and Planetary Science Conference. 27th: Abstract #1660. Full article available free at http://www.lpi.usra.edu/meetings/lpsc97/pdf/1660.PDF *

Ukstins Peate, I., van Soest, M. C., Wartho, J. A., Cabrol, N., Grin, E., Piatek, J., Piatek, J., Chong, G. 2010. A novel application of (U-Th)/He geochronology to constrain the age of small, young meteorite impact craters: a case study of the Monturaqui Crater, Chile [abstract 2161]. In Lunar and Planetary Science XLI. LPI Contribution No. 1533. Lunar and Planetary Institute, Houston, TX.

Valenzuela M. 2015. An Update on Meteorite Impacts in Chile and their Astrobiological Impications. Astrobio 2015, Santiago, Chile. http://www.eso.org/sci/meetings/2015/AstroBio2015/Talks/Tuesday/ValenzuelaM.pdf

Valenzuela M, Benado J. 2018. Meteorites and Craters Found in Chile: A Bridge to Introduce the First Attempt for Geoheritage Legal Protection in the Country. In book: Geoethics In Latin America, pp.103-115. https://www.researchgate.net/publication/323989601_Meteorites_and_Craters_Found_in_Chile_A_Bridge_to_Introduce_the_First_Attempt_for_Geoheritage_Legal_Protection_in_the_Country
ABOVE: The distribution of Atacamamites (Gattacceca et al., 2021). Google Earth Imagery.
The Atacamaite location is near a 'hole' or suspect crater (https://labodessavoirs.fr/blog-atacama/la-chasse-de-leurs-oeufs-paques/). An interesting nearby geo-form is also something I have heard from other sources. Clearly there is more information available, but I do not know any details. Looking on Google Earth there are numerous circular structures, but most of these are elevated volcanic peaks and not crater-like depressions. Could the crater be in the sea? Unlikely since this would require very shallow waters and the sea depth drops off rapidly in this region. Most likely the crater is on land, in proximity to the strewnfield and is hiding in plain sight.


To locate the crater, one could take a number of approaches:
  • Orientation of the strewnfield which is apparently northwest-southeast. This could be representative of a downrange ejecta ray or two butterfly rays sub-perpendicular to the direction of impact, with the crater in the middle. Note that Gattacceca et al. (2021) also records that a ‘few tens of similar-looking glass samples have been found sporadically about 40 km northwest of the strewnfield’. This is presumably close to the Very Large Telescope on Cerro Paranal in the Atacama Desert. One must also factor in water transportation and geological compatibility of the land (for instance sediments / ash / lava may result in burial of the Atacamaite-bearing surfaces elsewhere).
  • The number of specimens found in each square meter, the total weight of specimens found per square meter, the average and maximum weight of specimens in an area and so on and then use the R-squared regression technique. So long as there is a variable and so long as enough localities are sampled it should provide a direction in which to be searching, even if results are not precise due to lack of data.
  • I would get an Ar-Ar age for Atacamaites and Monturaqui Crater Glass, compare Sr-Nd, REE and so on to firmly eliminate Monturaqui Crater. I would geochemically test common rocks in surrounding regions and compare with Atacamaites.
  • Closely studying morphological character of Atacamaites in different areas of the strewnfield will help to determine where the crater is. Blocky specimens will be concentrated closer to the crater. Well-formed splash-forms will be at distance, with reference to Darwin Glass.

There is still much work to be done, not least in locating the crater. I hope that sufficient data was recorded by Gattacceca et al. (2021) to allow some R-squared regressions. I do think this would give an approximate crater location allowing for a focused geomorphological and field-based search for the source crater. Given the potential age of the structure it could be a lake, it could be heavily eroded, it could be filled with sediments, covered by volcanic ash, or beneath lava or a volcanic center (this is a volcanic region).
Atacamaites are a proximal impact glass, The impact site is likely within tens of kilometers of the Atacamaites. The crater may have been eroded, filled with sediments, volcanic ash, or covered by lava flows. The crater is most likely in the region of 0.5 to 14 km diameter and my hunch is 2 to 6 km. the impactor was and iron meteorite.

Atacamaite Source Crater

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