Iron, IC, octahedrite
High Re, Os, and Ir subgroup
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Found 1896
30° 20' N., 109° 59' W.

Two masses of 116 pounds and 20 pounds were found about 25 miles NW of Arispe, Sonora, Mexico. In 1898, another mass weighing 272 pounds was discovered 15 miles NW of Arispe and was sectioned and distributed to several museums. In the years following, many more masses were recovered along a 12-mile line from Mount San Antonio to Arispe. At least sixteen individuals with a total weight of over 1,500 pounds show that the Arispe shower was a significant one.

Arispe is a polycrystalline iron meteorite with distinctly oriented austenite crystals up to 25 cm in diameter. Troilite inclusions occur throughout, enveloped by schreibersite. The compositional trends are most consistent with formation from a residue of equilibrium partial melting. While distantly related to group IA irons, its lower Ni content of 6.1%–6.8% led to the establishment of a new group, IC. A unique feature present among group IC members is the occurrence of abundant cohenite and other carbon inclusions.

Radioisotope chronometry indicates an unusually long terrestrial age for the Arispe masses of 240,000 (±50,000) years. Based on corrected Hf–W isotope systematics, Kruijer et al. (2017) calculated the model age for core formation on the IC iron parent body at 0.3 (±0.5) m.y. after CAIs. This age overlaps within uncertainty the time of CAI formation and is the earliest differentiation, and thus the earliest accretion, of any parent body studied to date. Tornabene et al. (2021 #1531) revealed that isotopic data for most IC irons support a Re–Os isochron age of 4.57 b.y., which substantiates such an early accretion of the parent body.

W Isotope Dichotomy and Age Relative to CAIs
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Diagrams credit: Kruijer et al., PNAS, vol. 114, #26, p. 6713 (2017 open access link)
'Age of Jupiter inferred from the distinct genetics and formation times of meteorites'

Both highly siderophile element (HSE) data and W and Mo isotopic data were acquired by Tornabene et al. (2020 #2391; 2021 #1531; 2023) for each of the IC irons. It was demonstrated that the members of this group derive from a common parental melt and are related through varying degrees of fractional crystallization and solid–liquid mixing processes, with Arispe being the first to crystallize following ~7% fractional crystallization.

Tornabene et al. (2020, 2021, 2023) determined that the IC irons can be divided into two subgroups on the basis of whether the meteorites have high or low abundances of Re, Os, and Ir (see diagrams below). They posit that the two subgroups represent mixing between solids and liquids in various stages of evolution following a severe collision. The best fit model of Tornabene et al. (2023) that accounts for the IC group HSE concentrations has an initial core liquid composed of ~ 18 wt% S, 2 wt% P, and 0.03 wt% C. The high Re abundance subgroup represents mixtures of contemporaneously evolved solids and liquids, while the low Re abundance subgroup represents variable mixtures of early-formed solids present after <1% fractional crystallization in combination with evolved trapped metal persisting after 9% fractional crystallization, with the assumption that these latter meteorites are composed of >99% evolved liquid component. They derived a pre-exposure 182W model age for their IC group (excluding the anomalous Winburg) of 1.0 (±0.3) m.y. after CAIs. With respect to the disparate HSE data (µ182W isotope value) obtained for the anomalous IC iron Winburg, Tornabene et al. (2023) concluded that it either experienced late-stage alteration due to an impact event, or it represents a later metal–silicate segregation process that would date to 12.7 (±0.9) m.y. after CAIs. In addition, they used mass balance calculations to ascertain the mass of the IC parent body core, finding it to be ~25% of the mass of the IC parent body.

Highly Siderophile Element Data for IC Iron Subgroups
High abundance subgroup: Arispe, Nocoleche, Murnpeowie, Union County, Mount Dooling, Winburg
Low abundance subgroup: Bendego, NWA 2743, Etosha, Chihuahua City, St. Francois County, Santa Rosa

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Diagrams credit: Tornabene et al., 51st LPSC, #2391 (2020)

Using nucleosynthetic Pt isotope anomalies to correct for CRE-induced W isotope variations in iron meteorites, Spitzer et al. (2021) calculated new Hf–W differentiation ages for the iron chemical groups. It was determined that the IC irons have the oldest differentiation age among the established magmatic iron groups at 1.0 (±0.7) m.y. after CAIs (see diagram below).

Hf–W Ages Corrected for Nucleosynthetic Pt Isotope Anomalies
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Diagram credit: Spitzer et al. EPSL vol. 576, art. 117211 (2021, open access link)
'Nucleosynthetic Pt isotope anomalies and the Hf-W chronology
of core formation in inner and outer solar system planetesimals'

To learn more about the relationship between this and other iron chemical groups, click here. The specimen of Arispe shown above is a 39.6 g etched partial slice. A photo of the complete 1898 mass weighing 272 pounds is shown below.

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