Tombigbee River

Iron, IIG
(Compositionally linked to group IIAB)
standby for tombigbee river photo
Found 1859
32° 14′ N., 88° 12′ W. The Tombigbee River meteorite comprises a number of masses that fell over a 14 km ellipse aligned almost in a north–south direction (the largest masses to the south) in Choctaw County, near Jachin, Alabama. Although a meteor that was seen over most of Alabama in early 1848 was reported in newspapers from cities in the region of Jachin, the high level of oxidation exhibited by the Tombigbee iron masses make an association doubtful. The first of these masses, weighing 757 g, was discovered in 1859 by Mr. Ben Johnson. This first mass, along with five of the larger masses weighing ~ 15.0, 12.0, 9.2, 3.6, and 3.3 kg, were sold to the mineral dealer A. E. Foote in 1899.

Around this time, an additional ~3.6 kg mass was plowed up on the Daniel Plantation, one mile from the East Alabama Male College, Auburn. Due to the close similarities in chemical composition, structure, and weathering degree between Auburn and Tombigbee River, it was generally accepted that Auburn is a transported mass of Tombigbee River. The differences that do exist between the two irons—notably, the octahedral structure and the absence of schreibersite in Auburn—are consistent with the heterogeneous nature of this meteorite. Nevertheless, through correspondence between R.S. Clarke Jr., V.F. Buchwald, and J.T. Wasson in 1994 (copies in Min. Dept., NHM, London), it was decided that it was best to maintain a separate catalog entry for Auburn. Recent analyses of the Auburn mass (#443) in the Carleton B. Moore Meteorite Collection (Arizona State University) have demonstrated that this is likely a separate iron belonging to the chemical group IIAB (see the Auburn page for more complete details).

Tombigbee River is the most phosphorus-rich meteorite known, with abundant low-Ni schreibersite ribbons accounting for 9–15% by area. This high P content is consistent with late crystallization of dense, P-rich magma pockets following large degrees of fractional crystallization (>80%). Group IIG members are chemically most similar to those of the IIAB iron group, forming extensions to IIAB trends on element–Au diagrams. It has been proposed by Wasson and Choe (2009) that formation of IIG irons occurred inside isolated cavities which remained after crystallization of an evolved IIAB magma. The IIG irons eventually crystallized in a P-rich region of the lower layer of the IIAB core, while an immiscible and buoyant S-rich magma collected at the upper regions of the magma chamber. Elements such as Au and Ge were likely removed in the S-rich melt phase, while the low-Ni content of IIG irons is attributed to diffusion and redistibution of Ni out of metal and into schreibersite during an extended cooling history. The Ge-isotopic data were obtained by Luais et al. (2014), and they found it to be almost identical for both IIG and IIB metal, while a Ge content of 1.3 ppm and a δ74Ge of –3.4‰ was ascertained for schreibersite in Tombigbee River. Their Ge data support the formation history proposed by Wasson and Choe (2009).

Although Tombigbee River was initially classified as an ungrouped iron, it shared a similar composition with two other iron meteorites, Bellsbank and La Primitiva. As such, these three meteorites became known as the Bellsbank Trio. A fourth member of this iron grouplet, Twannberg, was subsequently recovered and the grouplet became known as the Bellsbank Quartet. In the year 2000, a fifth member of this grouplet, Guanaco, was recovered in the Atacama Desert, Chile, providing the requisite number of members needed to establish a new iron chemical group—The Bellsbank Quintet. John T. Wasson therefore proposed that this new iron group be designated IIG, its members representing the most Ni-poor (4.3%), schreibersite-rich meteorites of all iron groups. It is noteworthy that a sample of an iron meteorite from China, named Wu-Chu-Mu-Ch’in, which has a heterogeneous chemical composition and structure, includes a portion with trace elements that plot within the Bellsbank group IIG (Bartoschewitz, 2003). This iron portion also has the lowest Ni content (3.5%) of any known iron meteorite.

Most specimens in this group have a hexahedrite-like matrix consisting predominately of kamacite, but one Tombigbee River mass, which is lower in P and higher in Ni, shows a small area with a remnant coarse Thomson (Widmanstätten) structure. It was proposed by Vagn F. Buchwald that these irons be structurally classified as hexahedrite, transitional to coarsest octahedrite (H–Ogg). The masses have been terrestrially corroded but rare fusion crust and heat-affected zones are still visible. Neumann bands record more than one deformation event including the violent entry into Earth’s atmosphere. The photo above is a 6.64 g partial slice from a Tombigbee River specimen.

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