Iron, IAB complex, Mundrabilla duo
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Found 1911
30° 47′ S., 127° 33′ E. Initially, three small masses known as the Premier Downs masses were found between 1911 and 1918 in Western Australia. Other small specimens were subsequently found and given the names Loongana Station and Loongana Station West. In March of 1966, two larger masses of about 6 and 16 tons were found by geologists R. Wison and A. Cooney and described under the name Mundrabilla. This name was also given to the previously discovered masses without consideration to their former description under the name Premier Downs. The two larger masses, recovered only 180 m apart on the Nullarbor Plain just north of the Transcontinental Railway, can be matched up along similar sharp faces. In 1979, two more large masses weighing approximately one ton each were found by A. Carlisle about 20 km east of the 1966 location. In addition, over 500 small, knuckle-shaped specimens were found distributed along an east–west line of several miles, having a cumulative weight of over 24 tons.

Mundrabilla has a unique granular, polycrystalline structure composed of numerous, randomly oriented, 2–5 cm precursor taenite crystals separated by schreibersite, graphite, and troilite. The troilite forms veins or lenses several cm long along taenite crystal boundaries, accounting for up to 35 vol% of the meteorite. Rare, fine-grained, equigranular silicates similar to winonaite meteorites occur within these sulfide inclusions. Mundrabilla experienced rapid cooling from a melt phase, possibly the result of an impact that caused the disruption and reassembly of the parent body. Upon etching, an indistinct Thomson (Widmanstätten) structure is visible. Its extended terrestrial age has resulted in the removal of the fusion crust along with several mm of the surface, and might be partly responsible for the missing troilite between precursor taenite crystals which gives the large masses their knobby surface appearance.

Under the taxonomic revision of the IAB complex by Wasson and Kallemeyn (2002), both Mundrabilla and Waterville plot near the low-Au, low-Ni subgroup (sLL) with slight deviations. They also share very high (6×) FeS contents, and are therefore considered to be ungrouped members constituting a duo of the IAB complex; a designation of sLL anomalous is also considered correct. However, further investigation of the IAB complex irons with respect to HSE abundances was conducted by Worsham et al., 2016). Their data demonstrate that a significant difference exists between the HSE pattern for Mundrabilla and Waterville, with the latter being significantly different from typical IAB irons (see diagram below). standby for hse abundances diagram
Diagram credit: Worsham et al., GCA, vol. 188, p. 269 (2016)
Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group’
In contrast to the time of metal segregation and core formation on the IAB parent body at ~5 m.y. after Solar System formation, the separation of metal in Mundrabilla occurred ~8 m.y. later. At this time radiogenic heating had diminished and only impact-heating could plausibly explain large-scale melting events.

The photo above shows a typical ‘zoomorphic’ individual weighing 150 g consisting of three or four precursor taenite crystals. This mass was shaped by both selective ablation of troilite during atmospheric entry and terrestrial weathering processes. The photo below shows the cut surface of a large mass of Mundrabilla.

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Photo courtesy of S. Vasiliev

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See the ‘Meteorite Men’ episode Mundrabilla, Australia 21 December 2010, originally broadcast on the Science Channel and now available on YouTube.

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