IBITIRA


Eucrite-like basaltic achondrite, ungrouped
standby for ibitira photo
Fell June 30, 1957
20° S., 45° W. approx.

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. While each of these factors taken individually might not definitively resolve Ibitira from the established eucrites or other known basaltic meteorites (e.g., 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 compositional and isotopic similarities that exist between Ibitira, the eucrites, and the angrites suggest that they all likely formed from similar CV chondrite-like source material in relatively close proximity, but Ibitira and eucrites differentiated under reducing conditions while angrites differentiated under oxidizing conditions (Iizuka et al. (2015).

As presented by Sanborn and Yin (2014) [#2018], a Δ17O vs. ε54Cr diagram is one of the best available diagnostic tools for determining genetic (parent body) relationships between meteorites, constrained by the degree to which isotopic homogenization occurred on their respective parent bodies. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. Currently, a number of anomalous eucritic meteorites are known, including Ibitira, Bunburra Rockhole, Pasamonte, PCA 82502/91007, A-881394, NWA 1240, and EET 92023, each of which are resolved from typical eucrites and the HED parent body both isotopically and compositionally. The ureilites, generally considered to originate from a common parent body, have a relatively wide degree of variability in Δ17O, but a relatively narrow degree of variability in ε54Cr. By comparison, Sanborn et al. (2014) inferred that the similar degrees of variability that exist among these anomalous eucritic meteorites could likewise reflect a common origin from a single Vesta-like parent body distinct from typical eucrites (see diagram below). Several exceptions to this hypothesis have since been identified, and these include NWA 1240 which plots away from the common HED field, PCA 82502/91007 which is resolved from the other anomalous eucrites by both O-isotopes and pyroxene Fe/Mn ratio, A-881394 which Mittlefehldt et al. (2015) determined has significantly different oxygen isotopes, Ti/Al and Fe/Mn values, and bulk composition compared to HEDs, and EET 92023 which exhibits significant differences in O-isotopes, Cr-isotopes (Sanborn et al., 2016, #2256), and pyroxene composition compared to HEDs and other anomalous achondrites. EET 92023 shares similar O- and Cr-isotopes to A-881394 and Bunburra Rockhole indicating that they each formed within a common isotopic reservoir. Under the hypothesis that Δ17O values serve equally well as a discriminator compared to ε54Cr values, all of these anomalous meteorites could derive from numerous unique parent bodies distinct from Vesta.

standby for pasamonte o-cr diagram
Diagram adapted from Sanborn and Yin, 45th LPSC, #2018 (2014)

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 (Miyamoto et al. (2001). Ibitira experienced a very prolonged thermal annealing to a metamorphic grade of 5 (Takeda and Graham, 1991), equilibrating pyroxene and forming augite exsolution lamellae. Its igneous crystallization age based on the Pb–Pb age of pyroxene was determined to be 4.5561 (±0.0023) b.y., which is older than most all eucrites (Iizuka et al., 2013). With the determination of a more precisely calculated 238U/235U value, a slightly older Pb–Pb age of 4.55675 (± 0.00057) b.y. was obtained by Iizuka et al. (2014). This age is considered to represent the time of final equilibration during high-temperature metamorphism, probably soon after igneous crystallization. They suggest this stage of thermal metamorphism was initiated when the Ibitira source lava flow was buried by subsequent flows. Notably, the Pb–Pb age of Ibitira is virtually identical to its Mn–Cr age, calculated to be 4.5574 (± 0.0025) b.y. anchored to the D'Orbigny angrite. A subsequent impact heating event may be recorded by Ar–Ar chronometry ~4.49 b.y. ago, possibly representing the onset of a rapid cooling stage at ~850°C (Iizuka et al., 2014).

The Pb–Pb and Mn–Cr ages of Ibitira are identical to the those of the slowly cooled (sub-volcanic and plutonic) angrites such as LEW 86010, NWA 4801, and Angra dos Reis, measured to be ~4.557 b.y. old (Amelin et al., 2006; Lugmair and Shukolykov, 1998). Ibitira experienced a reheating event to a temperature of ~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. might 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 for a number of unbrecciated eucrites and cumulate eucrites (Bogard and Garrison, 2003). These similar ages are consistent with a major impact excavation at depth on the eucrite parent body, after which rapid cooling brought about the closure of the K–Ar chronometer. It is posited that this ~4.49 b.y. old event produced smaller daughter asteroids (Vestoids) from which unbrecciated and cumulate eucrites were eventually derived. However, this radiometric age data appears to contradict much of the diagnostic data presented in the paragraphs above and the presumption that Ibitira formed on a parent body separate from that of other eucrites. The CRE age for Ibitira was estimated through 81Kr-Kr dating by Shukolyukov and Begemann (1996) at 12.5 (±2.0) m.y., which straddles two CRE age clusters determined for eucrites.

A high temperature environment is indicated by the granoblastic texture as well as the extreme Ti-enrichment observed in Ibitira. Rapid cooling (50°C/yr) of a magma enriched in CO, CO2, and/or water (~50–200 ppm) occurred at considerable depth, accompanied by a rapid drop in pressure that promoted the formation of large (up to 0.5 cm diameter) vesicles constituting ~3–7 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.