CAMEL DONGA


Eucrite
Polymict, noncumulate
(Main Group–Nuevo Laredo trend)

standby for camel donga photo
Found January 1984
30° 19' S., 126° 37' E.

This polymict breccia was found on the Nullarbor Plain of Western Australia by Mrs. J. C. Campbell after she spotted a 503 g specimen from a moving vehicle while travelling cross-country. Eleven additional stones were recovered in July 1985 during a subsequent search of the site, located 200 m west of Camel Donga. Later search parties recovered more stones within the 1 km² area bringing the total known weight to over 2.92 kg.

Camel Donga is a product of fractional crystallization within a magma source and is composed of a mixture of pyroxene and plagioclase in a 3:2 ratio. The fine-grained gray matrix contains gabbroic and doleritic clasts. It contains an unusually high content of total Fe (18.6%), second only to that of NWA 4269 (23.18%). The groundmass of this meteorite has a high metallic iron content of ~2 wt% (~1 vol%) with grain sizes typically ranging from 5 to 10µm., while Fe is also present as finely dispersed particles in silicates. Because of the near absence of Ni (and other siderophile elements) in this metallic iron, it was hypothesized that it formed by in situ reduction of the ferrosilite (FeSiO3) component of pyroxene during in situ thermal impact metamorphism (Palme et al., 1988). Data from Hf/W ratios indicate that this thermal metamorphism occurred ~10–15 m.y. after mantle differentiation, long after the extinction of the major radiogenic elements. This scenario constrains the cause of the heating event to a major impact on Vesta (Kleine et al., 2004). A similar impact event is thought to have produced the pure Fe-metal in the eucrite NWA 6601 (Agee, 2011).

Conversely, in their study of Camel Donga, Warren and Isa (2015) found a lack of evidence for significant reduction in either the silicates or the Fe-oxides, of which the latter are considered to be the more susceptible to reducing agents. In addition, Warren et al. (2017) identified several cm-scale, metal-rich (12–17 vol%), ovoid silicate nodules (see photo below) and other nearby metal aggregates which are enriched in Ni relative to most other Fe-metal phases in the meteorite. These nodules are also enriched in trace siderophile elements and lack other evidence for in situ reduction. It was concluded that the metal nodules reflect the addition of an impactor component to the material that was precursory to Camel Donga. In view of these findings, it was proposed that the present composition of Camel Donga (in a similar manner to the eucrite NWA 5738) reflects a two-stage process of metasomatic alteration by a reducing fluid that originated as a metal- and volatile-rich carbonaceous-chondritic (e.g., CM-type) contaminant. It was speculated by Warren et al. (2017) that an initial impact-generated reduction process involving a S2- and CO-rich fluid derived from the exogenous nodules or other impactor material led to the production of troilite. Subsequent metasomatism involving H2O-rich fluids initiated a reaction with the existing troilite to produce the near-pure Fe-metal component in Camel Donga and NWA 5738 (see diagram below).

standby for camel donga nodule photo
Camel Donga ovoid nodule 11 mm wide in a 9g section; FoV 22 mm
standby for camel donga nodule formation diagram
Photo and diagram credit: Warren et al., MAPS, vol. 52, #4, (2017)
'Secondary-volatiles linked metallic iron in eucrites: The dual-origin metals of Camel Donga'
(http://dx.doi.org/10.1111/maps.12827)

Camel Donga has an absolute crystallization age based on the Pu–Xe chronometer of 4.507 b.y., and it has a cosmic-ray exposure age of 36.6 (±1.4) m.y., including it within one of the five common breakup events occurring 6 (±2), 12 (±2), 21 (±4), 38 (±8), and 73 (±3) m.y. ago. The pristine condition of these stones is evidence of a recent fall, and it is inferred from their shapes that a single fragmentation was followed by flight orientation of many of the individuals. The occurrence of regmaglypts, radial flow lines, and melt overflow is a common characteristic of many of the stones. The Camel Donga specimen pictured above is a 21.8 g individual with a cut face. The photo below shows the brecciated interior of this eucrite.

standby for camel donga photo