IBITIRA

A fall occurred at 5:15 P.M. and one 2.5 kg stone was recovered in the village of Ibitira, near Martinho Campos, in Minas Gerais, Brazil. This is a unique, unbrecciated, vesicular basaltic achondrite composed mainly of pyroxene in the form of pigeonite with exsolved augite, along with plagioclase and tridymite. Minor ilmenite, chromite, FeNi-metal, and troilite are present. Ibitira has been historically grouped with the Stannern trend eucrites according to compositional similarities, such as its plot on a TiO vs. FeO/MgO diagram and its major and trace element ratios. However, a recent in-depth petrologic analysis of Ibitira was conducted by D. Mittlefehldt (2005), the results of which have led to the proposal that Ibitira was formed on a parent body distinct from that of the HED suite basaltic achondrites (widely considered to be 4 Vesta).

Diagnostic data for Ibitira (Mittlefehldt, 2005; Lentz et al., 2007), which show significant deviations from representative eucrite data, includes higher Fe/Mn (34–36 vs. 30 ±2) and lower Fe/Mg ratios in low-Ca pyroxene, aberrant O-isotope ratios, high Ti/Hf ratios, a volatile-rich composition, and a low alkali element content with a correspondingly high Ca content in plagioclase. Although each of these factors individually may not be able to definitively resolve Ibitira from the established eucrites, or any of the other known basaltic meteorites—for example, O-isotope ratios for Ibitira are the same as those for angrites—when considered together, they are diagnostic for the formation of Ibitira on a unique parent asteroid. As deduced by Scott et al. (2008), the high degree to which impact gardening has occurred on Vesta would suggest that Ibitira-like lithologies should be present in other HED meteorites, which is not the case. The similarities that exist between Ibitira and the eucrites suggest that they all probably formed in relatively close proximity within the asteroid belt.

Ibitira is derived from in situ crystallization of residual melts within a magma ocean that was subsequently cooled at depth. Studies of the cooling rate and burial depth indicate that initial cooling down to 550°C proceeded at 0.02°C/yr at a depth approximating 30 m, 90 m, or 550 m, corresponding to a 50%-porous regolith, a compacted regolith, or a solid rock cover, respectively. Ibitira experienced a very prolonged thermal annealing to a metamorphic grade of 5 (Takeda and Graham, 1991), equilibrating pyroxene and forming augite exsolution lamellae. Ibitira has an igneous crystallization age of ~4.56 b.y., which is older than most all eucrites.

The Pb–Pb and Mn–Cr ages have been calculated to be as old as the youngest angrites, ~4.557 b.y. (Amelin et al., 2006; Lugmair and Shukolykov, 1998). Ibitira experienced a reheating event to ~1100°C when a large impact event excavated this material and formed a crater probably hundreds of kilometers wide. The Ar–Ar age of ~4.4858 b.y. may reflect this reheating event, which also resulted in the formation of a Ca gradient in the augite lamella, the recrystallization of plagioclase, and the formation of tridymite. Of possible significance is the existence of a tight clustering of Ar–Ar ages in common with that of Ibitira at 4.48 b.y. for a number of unbrecciated eucrites and cumulate eucrites (Bogard and Garrison, 2003). These like ages are consistent with a major impact excavation at depth on the eucrite parent body, after which, rapid cooling instituted the closure of the K–Ar chronometer. It is postulated that the 4.48 b.y. old event produced smaller asteroids (Vestoids) from which unbrecciated and cumulate eucrites were eventually derived. However, at the same time, this radiometric age data appears to contradict much of the diagnostic data presented above and the presumption that Ibitira formed on a parent body separate from that of other eucrites.

A high temperature environment is indicated by its granoblastic texture as well as its extreme Ti-enrichment. Rapid cooling (50°C/yr) of a CO–CO2-enriched magma (~50–200 ppm) occurred at considerable depths of 5–25 km, accompanied by a rapid drop in pressure, which promoted the formation of large (up to 0.5 cm diameter) vesicles constituting ~3 vol% of the rock (McCoy et al., 2003). The high Ca content of plagioclase indicates that water was present in the magma during vesicle formation, and H may have been a minor component of the vesicle-forming gas (Burbine et al., 2006). The subsequent mineral growth that occurred within these vesicles includes titanian chromite, ilmenite, whitlockite, and metallic iron. Tabular silica grains present in Ibitira, which are only present in eucrites with granoblastic textures, reflect high temperature metamorphic conditions; this is consistent with their proposed crystallization from the residual partial melt (Mayne et al., 2008).

The 485 g highly shocked and brecciated achondrite NWA 2824 shows many similarities to Ibitira, and the two may be related (Bunch et al., 2009). The specimen of Ibitira shown above is a 2.4 g partial slice exhibiting abundant vesicles.

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