Iron, IVA, octahedrite
standby for gibeon photo
Found 1838; known before
25° 20' S., 18° E. approx.

Although the native Nama people were aware of this iron meteorite before 1836, using the metal for spearheads and other weapons, it was not until this date that it was collected and described. Large masses were recovered in the Namibia Desert, Southwest Africa, in what is the largest strewnfield of any meteorite, covering an area of ~20,000 km². A total of over 21,400 kg have been recovered, with the largest mass of 650 kg on exhibit in the South African Museum in Cape Town. No impact crater is evident, but the severe twisting, over-folding, and cold-worked deformation of the masses attest to a violent atmospheric breakup, while the distinct regmaglypts are consistent with a long, high-velocity flight after breakup.

Gibeon is a polycrystalline octahedrite with a Ni content of 7.9% and a low P content, which exhibits a fine Thomson (Widmanstätten) structure. Along with other magmatic irons, the IVA parent body underwent differentiation, rapid cooling, and crystallization during the very early history of the Solar System, probably beginning within 1 m.y. of CAI formation. Formation of Gibeon occurred after ~30% crystallization of the magma, and a Pb–Pb age for a troilite inclusion in Gibeon was determined by Blichert-Toft et al. (2011) to be 4.544 (±0.007) b.y. Similarly, the IVA core segregation was established through the Pb–Pb chronometer from troilite in Muonionalusta to be 4.5651 (±0.0005) b.y., which is within 3 (±2) m.y. of its accretion (Blichert-Toft et al., 2010) and only ~2 m.y. after CAI formation. A correction was made to the 238U/235U value of troilite in Muonionalusta by Brennecka et al. (2016), which led to a revision in the Pb–Pb age to 4.5584 (±0.0005) b.y.

Rare silica veins (tridymite) present in both Gibeon and IVA iron Bishop Canyon were likely condensed from a cooling SiO-rich vapor that was emitted through reduction processes. Other IVA-related iron meteorites such as Steinbach, São João Nepomuceno, and Descoberto, all of which are likely source-paired, contain an abundance of orthopyroxene, tridymite, and troilite, and are considered to be products of igneous cumulates that were subsequently mixed with crystallizing metal during multiple impact events (Wasson et al., 2006; Ruzicka, 2014).

Earlier studies of the metallographic cooling history of the IVA parent body suggested that it had a diameter of 14–98 km (Haack et al., 1990), while another study placed the upper limit at 80 km (Wasson et al., 2006). Moskovitz and Walker (2011) argue that the cooling rates and the U–Pb closure age of the IVA parent body are consistent with a diameter of 50–110 km after the proposed catastrophic collision. The IVA asteroid is considered to have experienced a catastrophic break-up followed by gravitational reassembly at a time when core crystallation was nearly complete. This history is consistent with the inability to successfully model the elemental trends of the core material by simple fractional crystallization (N. Chabot, 2004), and it helps explain the widely disparate metallographic cooling rates that have been calculated, ranging from 6600°C/m.y. for the low-Ni, low-P subset, to 100°C/m.y. for the high-Ni, high-P subset. Alternatively, Moskovitz and Walker (2011) propose that the disparate cooling rates are the result of internal heating by the decay of 60Fe in an exposed core. Be that as it may, Wasson and Hoppe (2011) used a new method to determine cooling rates based on Co/Ni ratios in kamacite and taenite. They found no large variations in the cooling rates between two IVA irons—Bishop Canyon and Duchesne—even though they have metallographic cooling rates that differ by a factor of 25. However, Goldstein et al. (2012) argue that these new cooling rates obtained by Wasson and Hoppe (2011) reflect inadequate spatial resolution employing a flawed phase diagram and methodology.

Using a corrected growth mechanism for kamacite, Yang et al. (2008) calculated the cooling rate for Gibeon as 1500°C/m.y. for its Ni content of 8.04 wt%, in agreement with the finding of an inverse correlation between Ni content and cooling rate. The location of the Gibeon mass was calculated to have been ~10 km below the surface (Yang et al., 2011). A quenching phase during formation is consistent with the observed conversion of orthopyroxene into low-Ca clinopyroxene, although this conversion could be the result of impact shock forces as well.

A scenario proposed by Wasson et al. (2005, 2006) argues that the cooling rate estimates are correlated with the metal compositional range, and that this is best explained by a multiple-stage impact history for the IVA parent body as follows: 1) a high-velocity, high-temperature impact event onto a porous L/LL-like body melted and differentiated material within a deep crater; 2) dissociation of FeS and volatile loss of S, along with reduction of FeO and loss of O, produced an Fe-diluted, low-N metal magma; 3) this metallic melt drained into the core; 4) another high-temperature impact event injected a low-Ca pyroxene/silica melt into fractures in the crystallized, hot metallic core forming the Steinbach-like assemblages; 5) a later, less energetic impact event converted orthopyroxene to clinopyroxene.

Elaborate studies integrating fractional crystallization and thermal models led some investigators (Yang et al., 2006, 2008; Goldstein et al., 2006, 2009; Scott et al., 2007) to revise the formational history of the IVA parent asteroid in favor of a multi-collisional history different from that of Wasson et al. (2005, 2006). Based on the very wide range of cooling rates inferred from the Thomson structure and from the size of the high-Ni particles within cloudy taenite zones (measuring tens of nm in size), it was determined that the IVA irons cooled on an uninsulated metallic parent object measuring ~300 km in diameter. It was ascertained that the size of high-Ni particles within cloudy taenite zones were directly correlated with the bulk Ni concentration and with the widths of outer taenite rims (i.e., the tetrataenite region that separates the kamacite from the cloudy zone), and inversely correlated with the metallographic cooling rate (Goldstein et al., 2008).

Through measurements of the nm-scale high-Ni particles, Goldstein et al. (2009, 2010) concluded that this small metallic body, along with other small metallic objects, were derived from the differentiated core of a ~1,000-km-diameter protoplanetary body during a glancing collision with a larger body. During the collision, considered to have occurred 1–5 m.y. after CAI formation, the IVA body was stripped of all or most of its silicate mantle, while extricating volatile siderophile elements such as Ge and Ga. Steinbach-like assemblages would have been formed during such a collisional disruption.

Scott et al. (2011) envision a body that had original dimensions of >600 km when accounting for all of the differentiated layers, but which subsequently lost its silicate mantle in a glancing collision 1–2 m.y. after CAI formation. This object was then a molten metallic core 200–400 km in diameter which underwent rapid cooling. A second severe impact ~20 m.y. later produced a fragment >30 km in diameter or perhaps a rubble-pile asteroid. A final impact 400 m.y. ago delivered m-sized fragments to Earth.

The ~300-km, molten metallic body likely crystallized from the surface inwards toward the core, resulting in lower cooling rates for the more highly insulated, high-Ni, high-P iron subset. The cooling rate data of the IVA irons which have been studied place their crystallization at depths on the 300-km-diameter, uninsulated, metallic core fragment at between ~5 km (lowest Ni subset) and 90 km (highest Ni subset), the latter being about 60% of the distance to the center. On the other hand, Moskovitz and Walker (2011) combined the data of cooling rates, U–Pb closure age, and the estimated depth of the IVA Muonionalusta, and determined that the best fit was that of a 110-diameter core. They estimated that this iron formed at a core location situated 70% of its radius.

A second collision has been invoked to occur a few tens of m.y. after significant cooling of this 300 km-diameter metallic body, resulting in fragmentation and reassembly into a smaller body some tens of km in size. The resultant object was a heterogeneous rubble-pile comprising numerous components exhibiting a range of metallographic cooling rates. An alternative scenario has been developed by Wasson et al. (2006) which involves multiple non-disruptive collisions.

Ultimately, an impact event involving this small agglomerate body led to the delivery to Earth of the IVA iron meteorites we now have in worldwide collections. The Gibeon meteoroid is calculated to have had dimensions of 4 × 4 × 1.5 m and to have entered the Earth's atmosphere at a low angle of 10–20° from the horizon. Based on the Cl–Ar dating method, the CRE ages of the less insulated, low-Ni subset of IVA irons indicate that a catastrophic impact of the body occurred 420 (±70) m.y. ago. Secondary breakups affecting the more deeply buried, high-Ni material are reflected by two younger clusters at 255 (±15) m.y. and 207 (±13) m.y. ago. The O-isotope values of Steinbach and São João Nepomuceno are identical within analytical uncertainty to those of Gibeon and other IVA irons (Wang et al., 2004). Moreover, these O-isotope values are similar to those of L-LL chondrites, which suggests that an L-LL-like chondritic asteroid may be the precursor to the group IVA irons.

The Appendix III contains further details about the petrogenesis of the IVA iron group. The specimen shown above is a 6.7 kg individual (and its mirror image) with natural patina having a shape resembling a catcher's mitt. The photo below shows one of the most spectacular Gibeon individuals in existence—a 200 kg oriented, thumbprinted mass (photo courtesy of an anonymous collector).

standby for gibeon photo

Dr. Svend Buhl has written a comprehensive treatise on the Gibeon meteorites, "Gibeon Iron Meteorites: Discovery, History and Research", published on the Meteorite Recon website.