"Rock of the Month # 117, posted for March 2011" ---

The 54-kg main mass of the Bencubbin meteorite was discovered in 1930, almost completely buried in newly cleared ground on a wheat farm, ~240 km northeast of Perth, Western Australia. Simpson and Murray (1932) provided a detailed description. Bencubbin is an unusual meteorite with an estimated mode (mineral proportions in volume percent) of 47.8% Ni-Fe alloy, 23.1% enstatite, 20.2% olivine, 7.1% anorthite, 1.6% troilite and 0.2% chromite. The mean of specific gravity estimates was 5.32. Thus Bencubbin, being so metal-rich, was in a crude sense akin to stony irons (pallasites and mesosiderites), but the appearance was quite unique.

Eighty years later, Bencubbin is still very unusual, and so far all the expeditions in the cold and hot deserts of the world have added few similar examples. This distinctive find features in several compilations of meteorite lore (e.g, Brearley and Jones, 1998; Hutchison, 2004; Lauretta and Killgore, 2004). In fact, similar meteorites are classified as bencubbinites, a small "clan" with just six members, now subdivided into two (Weisberg et al., 1998, 2001; Krot et al., 2002, 2004). The greatest advance in this small sampling was arguably the fall of the Gujba meteorite in Nigeria on 03 April 1984 (Grossman and Zipfel, 2001). The two-fold division of the CB (Bencubbin-like) meteorites includes four CBa (Bencubbin, Gujba, Weatherford and NWA 1814) and two CBb (HaH 237, QUE94411) meteorites (Krot et al., 2002; Perron and Leroux, 2004).

Bencubbin has been revisited for its distinctive texture (Dehn and McCoy, 1998); the metal phase (Meibom et al., 1999; and its possible significance in terms of asteroidal parent bodies, of which our global meteorite collection may represent at least 135 (Meibom and Clark, 1999).Tiny, sub-micron diamonds have been detected in Bencubbin (Mostefaoui et al., 2001).

Metals in Bencubbin commonly form large (0.1 to 10 mm) compositionally-uniform clasts welded together with chondritic silicates by minor amounts of interstitial metal-silicate melt (Meibom et al., 1999). Campbell et al. (2002) examined the chemistry of the metal phase, and suggested that it might have formed by condensation of a metal-enriched gas formed by protoplanetary impact involving a metal-rich body. The matrix may have been at least partially melted, probably as a result of an impact (Perron et al., 2001). Glassy material in the matrix harbours small rounded vesicles (gas bubbles) in the 1-20 micron size range, mostly <10 µm. These abundant cavities are all <40 µm in diameter (Marty et al., 2000).

References

Brearley,AJ and Jones,RH (1998) Chondritic meteorites. In `Planetary Materials' (Papike,JJ editor), Min.Soc.Amer. Reviews in Mineralogy 36, chapter 3, 398pp.

Campbell,AJ, Humayun,M and Weisberg,MK (2002) Siderophile element constraints on the formation of metal in the metal-rich chondrites Bencubbin, Weatherford, and Gujba. Geochim.Cosmochim.Acta 66, 647-660.

Dehn,GA and McCoy,TJ (1998) The formational history of the Bencubbin meteorite: a macroscopic analyses. Lunar and Planetary Science 29, abstract 1193.

Grossman,JN and Zipfel,J (2001) The Meteoritical Bulletin, No.85, 2001 September. Meteoritics & Planetary Science 36 no.9, A293-322.

Hutchison,R (2004) Meteorites: a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press, 506pp.

Krot,AN, Amelin,Y, Russell,SS and Twelker,E (2004) Are chondrules in the CB carbonaceous chondrite Gujba primary (nebular) or secondary (asteroidal)? Meteoritics & Planetary Science 39, A56.

Krot,AN, Meibom,A, Weisberg,MK and Keil,K (2002) The CR chondrite clan: implications for early solar system processes. Meteoritics & Planetary Science 37, 1451-1490.

Lauretta,DS and Killgore,M (2004) A Color Atlas of Meteorites in Thin Section. Golden Retriever Publications / Southwest Meteorite Press, 301pp.

Marty,B, Perron,C and Fieni,C (2000) Vesicles in Bencubbin: evidence for shock-induced mobilization of heavy nitrogen and rare gases. Lunar and Planetary Science 31, abstract 1489.

Meibom,A and Clark,BE (1999) Evidence for the insignificance of ordinary chondritic material in the asteroid belt. Meteoritics & Planetary Science 34, 7-24.

Meibom,A, Petaev,MI, Krot,AN, Wood,JA and Keil,K (1999) Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition. J.Geophys.Res. 104 no.E9, 22053-22059.

Mostefaoui,S, El Goresy,A, Hoppe,P, Gillet,P and Ott,U (2001) In situ discovery of diamond in Bencubbin: evidence from Raman spectroscopy and cathodoluminescence. Meteoritics & Planetary Science 36 no.9, A141-142, Vatican City.

Perron,C and Leroux,H (2004) Fe-Ni metal in NWA 1814, the sixth Bencubbin-like meteorite: properties and origin. Meteoritics & Planetary Science 39, A82.

Perron,C, Fieni,C and Guilhaumou,N (2001) Glassy materials in Bencubbin: clues to its thermal history. Meteoritics & Planetary Science 36 no.9, A160-161, Vatican City.

Simpson,ES and Murray,DG (1932) A new siderolite from Bencubbin, Western Australia. Mineral.Mag. 23, 33-37.

Weisberg,MK, Prinz,M, Clayton,RN, Mayeda,TK, Sugiura,N and Zashu,S (1998) The bencubbinite (B) group of the CR clan. Meteoritics & Planetary Science 33, supplement, A166.

Weisberg,MK, Prinz,M, Clayton,RN, Mayeda,TK, Sugiura,N, Zashu,S and Ebihara,M (2001) A new metal-rich chondrite grouplet. Meteoritics & Planetary Science 36, 401-418.