Bunburra Rockhole meteorite, Nullarbor Plain, Western Region, South Australia, Australia

Eucrite, anomalous
Fall, 21 Jul 2007; 324 g

A bright fireball was recorded by observatories of the Desert Fireball Network. A fall position was triangulated and 2 fragments were recovered within 100 m of the predicted fall site. The smaller 150 g stone was oriented and completely covered by a fusion crust. When found the meteorite had experienced very little weathering. Dominant orthopyroxene (w. augite lamellae) and somewhat variable plagioclase (bytownite composition) account for ~90 vol% and are accompanied by fine-grained clasts. Silica is perhaps the most important minor phase (~4-5 vol%). Smaller amounts of chromite and ilmenite are also reported. Fe-Mn ratios in pyroxene (~31) are quite similar to those of other eucrites. The meteorite has experienced minimal preterrestrial shock and even less weathering. Pb-Pb dating provide an absolute age of ~4.1 billion years. However, Argon dating appears to reveal two strong impact events occurring ~ 3.6 billion years ago with perhaps an additional event ~1.4 billion years ago. The mineralogy, the impacts and the Fe/Mn ratios are quite unremarkable for a eucrite.

Quite remarkable, however, are Bunburra Rockhole's oxygen isotope ratios [ Δ17O‰ = -0.112 ± 0.042 (2σ)] placing it clearly outside the Eucritic norm. Cr-isotopes are also unusual. Bunburra Rockhole's atypical Aten-type orbit with perihelion (~.64 a.u.) well inside the earth's orbit is odd as well. Terminology for HED meteorites (Howardites-Eucrites-Diogenites) is evolving as Bunburra Rockhole now joins the small group of anomalous HED meteorites which are almost certainly not fragments of asteroid 4Vesta. The various anomalous meteorites, however, do not appear at this time to represent a coherent subgrouping as oxygen-isotope ratios, Fe/Mn ratios, and other indicators are somewhat disparate when determined in the different 'oddballs.'

Mineral List

8 entries listed. 3 valid minerals.

References

External Links