Unusual Meteorite Features (Ibitira with Vesicles)

Ibitira (Anomalous Basaltic Achondrite) - Partslice with Vesicles

The Ibitira meteorite fell at 5:15pm local time on June 30th, 1957 near the village of Ibitira in Martinho Campos county, Minas Gerais, Brazil. A single, mostly fusion crusted, flight-marked stone weighing 2500g was found on the Monjolo Farm in a small pit measuring approximately 25cm deep and 20cm across. Witnesses stated that there was a sound like thunder followed by a whistling noise. The fireball was visible up to 160km away.

Ibitira has traditionally been classified as a unique, unbrecciated, non-cumulate, vesicular eucrite. However, it is thought that Ibitira represents a fifth and distinctly different parent-body to other known basaltic eucrites from the HED/O (Vesta 4) group. Therefore, Ibitira has more recently been described and referred to as an anomalous basaltic achondrite.

Ibitira contains obvious vesicles that comprise approximately 5-7%vol of the stone. These vesicles range in size from about 0.2mm up to 2mm in diameter however the majority fall in the range of 0.3mm-0.5mm. Some areas have a lower density of vesicles than other more normal/higher density areas. The varying areas may be in very close proximity to each other which the first photograph below depicts well. The top portion of the partslice shows abundant vesicles while the lower portion of the specimen is almost devoid of them.

It is believed that the Ibitira material originally formed by rapid cooling of a magma flow between 2.5m to 20m thick. The rock may have formed as a sub-surface sill (approximately horizontal sheet of igneous rock intruded between two layers of older rock).

The vapour phase that deposited the vesicle minerals was in complete equilibrium with the matrix materials. This is evident through the many studies which have shown the similarity of Rare Earth Element (REE) abundances between the vesicle minerals and matrix. It indicates that the vapour formation that deposited the vesicle minerals was not active and fast but rather one that allowed equilibrium without altering the REE micro-distributions in the matrix minerals.

One theory suggests that the creation of Ibitira's vesicles is the result of volatile exsolution (unmixing) from the parent magma. Ibitira may have crystallised from a super-heated total impact melt early in the solar system’s formation, with the vesicles being formed by the release of volatiles of the common rock elements (Na, K, Si, etc).

The crusted Ibitira partslice below weighs 2.275g and is part of the Meteorites Australia Collection (MA.06.0017).

Rare Earth Element abundances in vapor deposited minerals in Ibitira.
N. A. Heim, M. Wadhwa, A. M. Davis (181kb)

Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission. David W. Mittlefehldt (2.04MB)

Vapor deposited mineral assemblages in vesicles of the eucrite Ibitira.
M. Wadhwa & A. M. Davis. (788kb)

Ibitira (Anomalous Basaltic Achondrite) - 2.275g Partslice
(Click image to enlarge in new window.)

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