Toluca (meteorite)

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Toluca meteorite

The Toluca meteorite was discovered in the Toluca Valley ( Jiquipilco , Mexico ) before 1776 . The metal of the iron meteorite was used by the Indians living there for the production of agricultural tools.

Research history

The earliest written evidence of iron meteorites found in the Toluca area comes from the "Gazeta de México", one of the oldest newspapers in Mexico, from December 15, 1784, in which it is already reported that the inhabitants of the region around "Xiquipilco" (today Jiquipilco) only use chunks of solid iron in various shapes and sizes for the production of their agricultural implements. The year 1776, which is given in the specialist literature and also in the Meteoritical Bulletin Database , goes back to a note published in 1831 by Jose Antonio Alzate Ramirez, in which he reports that he had already observed with his own eyes how the forge of "Xiquipilco" saw the pieces of iron as early as 1776 that the locals were specifically looking for, especially at the beginning of the rainy season.

Until the first half of the 19th century, only a few specimens came to Europe or the USA. This only changed when Adam August Krantz initiated a targeted search in 1856, during which 69 smaller fragments with a total mass of 49.5 kg could be recovered. Further searches followed and in 1975 Vagn Buchwald estimated the total mass of the fragments recovered from the extensive litter field at around 2.8 tons.

Mineralogy, Geochemistry and Classification

The basic mass of the Toluca meteorite consists of kamacite and taenite , which form clear Widmanstatt structures . Buchwald gives the width of the Kamacit bars as 1.40 ± 0.20 mm. The meteorite thus falls into the structural class of the coarse octahedrites (Og). The spaces between the Widmanstatt structures are filled with plessit . Haxonite and daubréelite also occur in small quantities . The matrix often contains large, bulbous inclusions of troilite and / or graphite . Rare silicate fragments within these tubers essentially consist of olivine , diopside and smaller amounts of plagioclase , orthoclase , zirconium and cosmochlor may also occur occasionally. Within or on the edge of troilite / graphite nodules native copper can sphalerite , Alabandin , schreibersite , cohenite and Djerfisherit occur. For the two minerals haxonite and cosmochlor, the meteorite is also a type locality .

Chemical composition

  • Main and secondary elements:
    • Fe: ~ 91%
    • Ni: 8.14%
    • Co: 0.49%
    • P: 0.16%
    • S: 0.7%
  • Trace elements (selection according to the geochemical classification according to Wasson & Kallemeyn , 2002):
    • Ge: 245 ppm
    • Cu: 170 ppm
    • Ga: 68.9 ppm
    • Cr: 23 ppm
    • As: 16.5 ppm
    • Pt: 5.7 ppm
    • Ir: 2.47 ppm
    • Au: 1.718 ppm
    • W: 0.83 ppm
    • Sb: 380 ppb
    • Re: 250 ppb

According to the classification scheme of Wasson & Kallemeyn, 2002, the Toluca meteorite falls accordingly into the group of IAB meteorites and within this group into the sLL subgroup (“subgroup Low-Au / Low-Ni”). Of this subgroup ("Iron, IAB-sLL"), only 32 cases are known so far (as of August 2018).

origin

The extensive sample material enabled numerous investigations and analyzes, which not only led to a detailed geochemical and mineralogical- petrological characterization, but also allow conclusions to be drawn about the history of the origin of the Toluca meteorite.

IAB meteorites do not originate from the metallic core of a mother body, which is completely differentiated into a metallic core and a silicate mantle / crust . On the other hand, both the trace element content and the frequent silicate inclusions speak for a differentiation that is only just beginning. The silicate inclusions can also be associated with Winonaites , which may have the same origin. Possible heat sources for this beginning differentiation are impacts on one or more very similar original bodies or the decay of short-lived isotopes , such as 26 Al or 60 Fe, in a larger, single original body.

In 2018, Alison C. Hunt and co-authors publish new geochronological age data for various IAB meteorites, including fragments of the Toluca meteorite. The hafnium-tungsten method used allows conclusions to be drawn about the point in time at which the differentiation of iron melt and silicates occurs. The results showed that IAB meteorites of the main group (MG), the subgroups sLL (Toluca) and sLM (“subgroup Low-Au / Medium-Ni”) as well as some other representatives from the IAB complex had almost the same hafnium-tungsten age exhibit, which supports the hypothesis of a single, larger body of origin for these the meteorites.

Hunt's authors supplement their age data with some model calculations on the heat development of the original body and draw the following picture: About 1.4 million years (Ma) after the first condensates ( CAIs ) formed from the cooling protoplanetary nebula , a planetesimal formed with a Diameter of 60 km or a little more. The body was big enough that the heat caused by radioactive decay about 6 Ma after the CAIs had formed could initiate a differentiation between molten metal and silicates. The body, however, was not big enough for it to be completely separated into a metallic core and a silicate coat. Already about 10 Ma after the formation of the CAIs, the molten metal within the silicate shell began to solidify again before it could reach a metal core that was still liquid at that time. About 10-14 Ma after the CAIs had formed, part of the molten metal was still liquid at this point in time, the original body was hit by a severe impact that largely shattered it. The debris, however, remained gravitationally bound and formed a new planetesimal with a strongly mixed, heterogeneous composition. In the case of the Toluca meteorite, this led to the special composition of largely metallic melt with the frequent sulphide / graphite nodules and their inclusions from silicate fragments.

Web links

Commons : Toluca meteorite  - collection of images, videos and audio files

Individual evidence

  1. ↑ "Æ": Xiquipilco In: Gazeta de Mexico - Miercoles 15 de Diciemb. de 1784 , N. 25, pp. 201–202, 1784. (digitized version) (Spanish)
  2. ^ A b The Meteoritical Society - International Society for Meteoritics and Planetary Science: Meteoritical Bulletin Database. (Database entry for Toluca) Retrieved August 13, 2018
  3. ↑ JA Alzate Ramirez: Gacetas de Literatura de Mexico. Volume 2, p. 380ff, 1831. (reading sample) (Spanish)
  4. ↑ A. Krantz: About Meteoreisen from the Toluccathal in Mexico. In: Annals of Physics and Chemistry , Volume 101, pp. 152–153, 1857. (reading sample)
  5. ↑ a b c d e f V. F. Buchwald: Handbook of Iron Meteorites - Their History, Distribution, Composition and Structure. Volume 3, pp. 1209ff, University of California Press, 1975. (excerpt)
  6. ^ VF Buchwald: The mineralogy of iron meteorites. In: Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences , Vol. 286, No. 1336, pp. 453-491, 1977. (digitized version )
  7. ↑ Mindat - type locality Toluca meteorite, Jiquipilco (Xiquipilco), Mexico
  8. ↑ a b c d e f g h i j k l m n o J. T. Wasson & GW Kallemeyn: the IAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts . In: Geochimica et Cosmochimica Acta , Vol. 66, Issue 13, pp. 2445–2473, 2002. (abstract)
  9. ^ A b c G. Benedix, T. McCoy, K. Keil & S. Love: A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-Winonaite parent body. In: Meteoritics & Planetary Science , Vol. 35, pp. 1127–1141, 2000. (digitized version)
  10. ↑ G. Benedix, T. McCoy, D. Bogard & D. Garrison: A petrologic and isotopic study of winonaites: evidence for early partial melting, brecciation, and metamorphism. In: Geochimica et Cosmochimica Acta , Vol. 62, Issue 14, pp. 2535-2553, 1998. (abstract)
  11. ↑ a b c A. C. Hunt, DL Cook, T. Lichtenberg, Ph. M. Reger, M. Ek, GJ Golabek & M. Schönbächler: Late metal – silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modeling. In: Earth and Planetary Science Letters , Vol. 482, pp. 490–500, 2018. (online)