TOLUCA


Iron, IAB complex, sLL subgroup
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Found 1776, known earlier
19° 34' N., 99° 34' W.

Many large masses were found near Xiquipilco, Mexico, the largest of which weighed 300 pounds. Recently, a taxonomic revision was proposed by Wasson and Kallemeyn (2002) that includes iron meteorites from the IAB-IIICD group, along with numerous IAB-related meteorites. On a Ni–Au diagram, Toluca and other similar irons compose a low-Au, low-Ni subgroup (sLL).

The I–Xe closure age of Toluca was determined by Pravdivtseva et al. (2009) for both high-Mg and low-Mg pyroxenes, the two closure times differing by 8.5 (±4.4) m.y. An absolute closure age based on the Shallowater standard was calculated to be 4.5605 (±0.0024) b.y. for high-Mg pyroxenes and 4.5520 (±0.0037) b.y. for low-Mg pyroxenes, a range similar to that of other IAB iron silicates. The earlier closure age may be commensurate with a catastrophic disruption of the IAB parent body. The research team also determined the cooling rate of Toluca following parent body breakup and reassembly. This was calculated as a function of the difference between the crystallization temperatures and closure ages of the two pyroxenes to be 14.5 (±10.0)°C/m.y.

A comparative study of the IAB iron main group (MG) and sLL subgroup by Worsham et al. (2013, 2016, 2017) demonstrated through Mo isotope compositions that both groups derive from a common parent body that was initially chondritic. They verified through HSE data that irons from these two groups crystallized from distinct parental melt pools, with the smaller sLL formation event occurring 0–3 m.y. after the MG formation event. Moreover, the observed fractionations are not the result of fractional crystallization, but instead, most likely involved crystal segregation and other processes related to the respective impacts. Further evidence for formation in distinct melt pools among the IAB iron groups was found through cooling rate studies correlating the cloudy zone particle size with the metallographic cooling rate (Goldstein et al., 2013). Moreover, the slow cooling rates determined for the IAB irons and other meteorite groups containing silicate assemblages (e.g., pallasites and mesosiderites), were found to be inconsistent with the faster cooling rates attributed to those iron groups that underwent fractional crystallization in cores lacking insulating silicate mantles (e.g., IIIAB, IVA, and IVB).

Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] and the ungrouped Caddo County [Udei Station grouplet] and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIs. They contend that a catastrophic breakup and reassembly occurred during which different silicate lithologies were mixed. Based on the CRE-corrected W data, Worsham et al. (2017) derived a segregation age corresponding to 3.4 (±0.7) and 5.0 (±1.0) m.y. after CAIs for the MG and sLL subgroup, respectively. They argue that a breakup and reassembly event would have also mixed different metal lithologies together, and would have equilibrated the W systematics of the MG and sLL subgroup. The top schematic diagram below is the model of Hunt et al. (2018), which shows the early history of the 120(+)-km-diameter IAB parent body based on constraints provided by the timing of metal segregation. The schematic diagram beneath that one shows the impact-generated melt model of Worsham et al. (2017).

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Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open access link)
'Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling'
(https://doi.org/10.1016/j.epsl.2017.11.034)

standby for iab iron formation diagram
Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 164 (2017)
'Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex'
(https://doi.org/10.1016/j.epsl.2017.02.044)

Based on the similar silicate textures, reduced mineral chemistry, and O and Mo isotopes, it is presumed that the winonaites and the IAB complex irons originated on a common parent body. Utilizing a Ge/Ni vs. Au/Ni coupled diagram, Hidaka et al. (2015) determined that FeNi-metal in the winonaite Y-8005 plots in the field of the sLL subgroup of the IAB complex irons. Worsham et al. (2017) also demonstrated that the Mo isotope data for the two winonaites they studied, Winona and HaH 193, attest to a common parent body for winonaites and MG/sLL irons. Moreover, the metal in Y-8005 retains a near chondritic composition likely representative of the precursor material of the parent body. In view of these findings, Hidaka et al. (2015) suggest that the sLL subgroup rather than the MG represents the primitive metal of the IAB–winonaite parent body, with the MG possibly representing a partial melt of the sLL subgroup.

Toluca has a high CRE age of 600 (±150) m.y. (Chang and Wänke, 1969). The silicate ureyite (NaCrSi2O6) has been found as a rare occurrence in Toluca (Frondel and Klein, 1965). Further information on the formation of the IAB iron complex can be found in the Appendix, Part III. The specimen of Toluca shown above and below is a 2003 Harvey Award—"New Technology Award"—which was presented in recognition of the Meteorite Studies website.

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