GUJBA

At 6:30 in the evening, a bright fireball approaching from the west was seen and heard by local residents. The cone-shaped mass that landed in a corn field near the village of Bogga Dingare in Yobe, Nigeria, was estimated to have weighed ~100 kg, but most of the mass was broken up into small pieces and dispersed. Gujba is a primitive, polymict, chondritic breccia, the first fall among the bencubbinite group. It contains 0.4–8 mm-diameter, rounded, metallic globules (~41 vol%), and 0.8–15 mm-diameter silicate globules (~20 vol%), cemented together by a dark-colored, silicate-rich, impact melt matrix composed of mm-sized fragments of both silicate and metal globules (~39 vol%).

Metal globules may contain up to ~1 vol% troilite, which is positively correlated with the abundance of volatile siderophile elements. Fractionation of siderophile elements in Gujba was controlled by volatility rather than by oxidation/sulfidation processes or magmatic crystallization (Krot et al., 2002). Siderophile element correlations are inconsistent with a nebular condensation model. It is generally assumed that a protoplanetary impact involving an undifferentiated, probably carbonaceous chondritic body, gave rise to a vapor cloud with high enough partial pressures to generate a metal-enriched gas. The metal globules then condensed as liquids from this gas and were sorted by size and density, thereby establishing the high metal/silicate ratio of the group. The CB group reflects a sequence of increasingly lighter Fe isotopes from Gujba through HaH 237 to Isheyevo (Zipfel and Weyer, 2006). This wide Fe isotopic range provides further evidence of a formation within an impact vapor plume rather than in a nebula setting.

In a nm-scale study of Gujba, a two-phase (kamacite and taenite) metal particle was observed comprising ~30 individual grains that demonstrate a reheating episode occurred at temperatures of ~675°C (Goldstein et al, 2011). It is estimated that a subsequent cooling to ~550°C occurred within a time period of a month. A similar metal particle having a similar thermal history was found in the CBb bencubbinite HaH 237. These metal particles are consistent with an origin in an impact plume, followed by accretion to a secondary parent body, where they experienced impact-associated secondary heating. The injection of impact melt matrix into metal and silicate components may also have occurred at this time.

Hydrocode modeling utilizing a homogeneous nucleation theory demonstrates that the very high densities and temperatures that would lead to the formation of mm- to cm-sized metal globules are consistent with an impact vapor plume origin rather than a nebular origin (Anic et al., 2005). A high velocity collision is most likely to produce those conditions conducive to producing the particle size that exists in Gujba. An alternative model has been described whereby the metal was melted to form globules, while S and volatile siderophiles were subsequently evaporated out. The metal globules have varying Ni contents and exhibit quench textures (Rubin et al., 2003), and since no diffusion has occurred between globules in contact with each other, it can be inferred that they were accreted at cold temperatures after being isolated from the hot condensation region.

The silicate globules in Gujba exhibit skeletal olivine textures, contain no FeNi-metal or troilite, and have low concentrations of volatile elements, features indicative of quenching from a molten state, i.e., condensation from a hot, impact-generated vapor plume (Krot et al., 2004). CAIs have been found in some bencubbinites including HaH 237, QUE 94411, and Gujba, and the transitional member Isheyevo, and are thought to have condensed from the vapor plume along with the metal and silicate globules.

Raman spectra have identified the first carbonaceous chondrite occurrence of several high pressure phases within barred olivine fragments and the matrix components of Gujba; these include majorite garnet, majorite-pyrope solid solution, and wadsleyite, along with minor grossular-pyrope solid solution and coesite (Weisberg and Kimura, 2010). These high pressure phases formed either from solid-state transformation of pyroxene or crystallization from an impact melt during a heterogeneous, planetesimal wide impact shock event reaching minimum pressures of ~19 GPa and temperatures of ~2000°C. The investigators argue that these high pressure phases are inconsistent with the subsequent formation of chondrules within an impact plume since at such high temperatures these phases would be rapidly back-transformed to their low temperature polymorphs. Moreover, the measured cooling rates of chondrules (ave. 100°K/hr) are much too slow than that at which shock veins with high pressure polymorphs would survive (~1000°K/hr). Therefore, they determined that the barred chondrules and metal in CB chondrites were formed prior to the impact event which produced the high pressure polymorphs.

The bencubbinites constitute a small group having similar oxygen and nitrogen isotopic compositions as well as similar petrologic characteristics. They have highly reduced silicates, bulk metal abundances of 60–70 vol%, Cr-bearing troilite, metal with near solar Ni/Co ratios, and similar elemental abundances. Among chondrite groups, the bencubbinites show a significant enrichment of 15N, with Gujba having an intermediate content within the group. The bencubbinites have been divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate globules, and those with mm-sized globules, respectively. Further information on the formation of bencubbinites can be found on the Bencubbin and Isheyevo pages.

Based on Pb–Pb isotopic systematics, the chondrules in Gujba (CBa) and HaH 237 (CBb) formed simultaneously 4.5627 (±0.00025) b.y. ago, reflecting a younger age than for other chondrite groups. Employing I–Xe data from Gujba and HaH 237, Gilmour et al. (2009) made the conversion to an absolute age based on the Shallowater standard (here newly revised to 4.5623 [±0.0004] b.y.) to yield a closure age of 4.5642 (±0.0006) b.y. It was further elucidated that the I–Xe age of CB chondrules was older than the Pb–Pb age, and therefore, the absolute age cannot reflect the actual time of formation; rather, it attests to post-formation processing of chondrules.

The CRE age of Gujba (26 ±7 m.y.) is identical within uncertainties to that of Bencubbin (27.3 m.y.), and both have similar noble gas concentrations (also similar in some respects to the enstatite chondrites; Nakashima and Nagao, 2009), which attests to a common ejection event on their parent body. However, while the metal and silicate globules in Gujba are mostly complete, undistorted spheres, those in Bencubbin and Weatherford are fragmented and distorted. Gujba and other CB chondrites exhibit multiple characteristics that are consistent with a severe shock subsequent to its formation, including melting, brecciation, and deformation. The presence of certain high-pressure phases in Gujba—majorite and wadsleyite, produced by the conversion of low-Ca pyroxene and olivine, respectively—attests to the occurrence of a significant shock event of ~19 GPa at 2000°C (Weisberg and Kimura, 2004) consistent with a shock stage of S2. As in Bencubbin, shock-associated structures identified in Gujba include stishovite, amorphous to poorly graphitized carbon, ordered graphite, rounded to euhedral diamonds, nanodiamond clumps, and rare bucky-diamonds, along with carbonaceous nanoglobules (Garvie et al., 2011).

The CB, CH, and CR chondrites constitute the CR clan, comprising groups which likely formed in the same isotopic reservoir under similar conditions in the solar nebula; current evidence argues for an origin of the metal-rich carbonaceous chondrites in a common collision between planetary embryos (Krot et al., 2009). The Gujba specimen pictured above is an 18.1 g polished slice sectioned from a 282 g fragment that was originally purchased in 2000 in Gidan Wire, Nigeria. See also a magnificent 82.2 g full slice of this special bencubbinite. The photo below shows an awesome 2,365 g end piece, part of the J. Piatek Collection, which was sectioned from a 3,440 g complete Gujba mass.

Photo courtesy of Dr. J. Piatek