Violent solar system history uncovered by WA meteorite

MEDIA RELEASE

Friday 8 August 2014

Curtin University planetary scientists have shed some light on the bombardment history of our SOLAR SYSTEM by studying a unique volcanic meteorite recovered in Western Australia.

Captured on camera seven years ago falling on the WA side of the Nullarbor Plain, the Bunburra Rockhole Meterorite has unique characteristics that suggest it came from a large asteroid that has never before been identified.

Associate Professor Fred Jourdan, along with colleagues Professor Phil Bland and Dr Gretchen Benedix from Curtin’s Department of Applied Geology, believe the meteorite is evidence that a series of collisions of asteroids occurred more than 3.4 billion years ago.

“This meteorite is definitely one-of-a-kind,” Dr Jourdan said.

“Nearly all meteorites we locate come from Vesta, the second largest asteroid in the SOLAR SYSTEM. But after studying the meteorite’s composition and orbit, it appears it derived from a large, unidentified asteroid that was split apart during the collisions.”

The research team dated the meteorite with the argon-argon technique, a well-known method for DATING impact crater events, to offer a glimpse of the asteroid’s impact history.

They obtained three series of ages indicating that the meteorite recorded three impact events between 3.6 billion and 3.4 billion years ago.

“These ages are pretty old by terrestrial standards, but quite young for a meteorite since most are dated at 4.57 billion years old, when the SOLAR SYSTEM began,” Dr Jourdan said.

“Interestingly, the results also showed that not a SINGLE impact occurred on this meteorite after 3.4 billion years ago until it fell to Earth in 2007.

“The same impact history has also been OBSERVED from meteorites originating from Vesta with any impact activity stopping after 3.4 billion years ago.

“Obtaining similar information from two large, yet distinct asteroids is an exciting discovery as it confirms some of the bombardment history of our SOLAR SYSTEM.”

Dr Jourdan said the reason for impacts stopping after 3.4 billion years ago could have been from the asteroids being too small in size to be a target for collisions, or protected by regolith, a thick blanket of cushiony powder usually found at the surface of asteroids.

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The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic OBSERVATIONS constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine 40Ar/39Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n = 7; P = 0.53) and 3544 ± 26 Ma (n = 2; P = 0.54), respectively. Based on these results, additional 40Ar/39Ar data of fusion crust fragments, argon diffusion modelling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ∼3.64 Ga by a medium to large impact event. The data imply that this impact generated high enough ENERGY to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb–Pb age. We also conclude that a complete 40Ar∗ resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analog (Lonar crater) shows that the time–temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighbouring part of the same target rock at ∼3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ∼3.42 Ga, a third smaller impact excavated parts of the ∼3.64 Ga and ∼3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggests that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat-absorbing regolith blanket which, when combined with the diminishing frequency of large impacts in the SOLAR SYSTEM, protected Bunburra from subsequent major heating events. Finally we note that the total (K/Ar) resetting impact event history recorded by some of the brecciated eucrites (peak at 3.8–3.5 Ga) is similar to the large impact history recorded by the Bunburra Rockhole parent body (ca. 3.64–3.54 Ga; this study) and could indicate a similar position in the asteroid belt at that time.

Quelle: Curtin University