BILANGA


Diogenite
Orthopyroxenite
(≥90 vol% orthopyroxene)

standby for bilanga photo
Fell October 27, 1999
12° 27' N., 0° 05' W.

At 10:30 A.M. local time a shower of rare diogenite meteorites fell in the villages of Bilanga Yanga and Gomponsago, Burkina Faso, Africa. The total weight is more than 25 kg, including a 7.5 kg oriented mass with centrally radiating flow lines. Mineralogical analyses performed at the Institut für Planetologie, Münster, Germany, has determined that Bilanga is a diogenite.

Bilanga consists of calcium-poor orthopyroxene crystals broken up in a brecciated matrix containing aggregates of diopside and plagioclase, associated with chromite, troilite, and silica. These assemblages represent cumulates formed by late-stage fractional crystallization of a melt, followed by slow cooling and thermal metamorphism. Trapped melt inclusions consisting of these same components also occur between orthopyroxene grains, likewise sampling a late-stage intercumulus melt. The silica-rich mesostasis is enriched in KREEP. A single grain of native copper has been observed adjacent to a large tetrataenite grain, and plagioclase-rich clasts occur which are thought to represent an exotic rock type, possibly associated with the silica-rich mesostasis (Domanik et al., 2004). Non-cumulate textured zones which exhibit localized shock-melting and rapid recrystallization are present.

Oxygen-isotopic studies carried out at the Enrico–Fermi Institiute, University of Chicago, Illinois, indicate that Bilanga is closely related to the Shalka diogenite, and Shalka has been found to have similarities to Tatahouine. However, while both Shalka and Tatahouine have a similar CRE age coinciding with a major HED peak at 38 m.y., Bilanga has a CRE age of 49 m.y., coinciding with a lesser HED peak. Since this peak at ~50 m.y. is shared by the Garland and GRO 95555 diogenites, the Macibini eucrite, and possibly the Yurtuk howardite, the CRE age of Bilanga strengthens the case for an impact event on the HED parent body ~50 m.y. ago.

A different relationship has been shown to exist among Bilanga, GRA 98108, MET 00425, and Manegaon. These four diogenites are Sc-poor and Sm-rich, and their pyroxenes have the highest magnesium contents among the diogenites. Correlations among these parameters are consistent with a co-magmatic origin within an igneously fractionated cumulate pile. An alternative viewpoint was presented by Barrat et al. (2010) based on the fact that orthopyroxenes in Bilanga exhibit similar REE patterns and light REE depletions as the Johnstown, Roda, A-881548, and Dhofar 700 diogenites, possibly constituting a subgroup. The investigators argue that this group of diogenites all having very low Eu/Eu* ratios were contaminated by <10% of a low degree (<5%) eucritic crustal melt phase associated with a very large Eu anomaly. This scenario would be consistent with diogenites forming concurrent with or later than the formation of eucrites.

For more details on the formation of diogenites visit the Johnstown page. To see an alternative classification system for the diogenites and dunites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. A petrographic thin section micrograph of Bilanga can be seen on J. Kashuba's page. In the photo above, the specimen on the right is a 5.5 g partial slice and on the left a 4.4 g fragment showing two parallel flow ribs. The top photo below shows the 7.5 kg main mass, and below that is its cut face. The final photo below shows a Bilanga specimen with parallel shock veins rarely seen in this diogenite.

standby for bilanga photo
standby for bilanga photo
Photos courtesy of Michael Casper
standby for bilanga photo
Photo courtesy of the Martin Altmann Collection