Posted on Leave a comment

Bellsbank Quintet: A New Iron Group

THE BELLSBANK QUINTET:

The Consummation of a New Iron Group—IIG The five iron meteorites displayed on this page, which constitute the newly designated group IIG, sharing the charateristics of low-Ni, high-P hexahedrites, are presented here courtesy of the Dr. J. Piatek Collection.


Tombigbee River

standby for bellsbank photo

An iron mass was found in 1859 in western Alabama, USA, followed in subsequent years by the recovery of five additional masses; these six iron masses had a combined weight of 43.8 kg. In 1867, an extensively oxidized 3.63 kg mass was plowed up 250 km east of the Tombigbee River find. The meteorite was given the name Auburn, and although historically considered to be a transported piece of Tombigbee River, it was demonstrated by Hilton and Walker (2019) that it is likely a separate iron belonging to the IIAB group. Tombigbee River was initially classified as an ungrouped iron, but since it was shown to have similar compositions to two later found iron meteorites, La Primitiva and Bellsbank, these three meteorites became known as the Bellsbank Trio. The photo above shows a 117.2 g slice of this rare fall, acquired from the United States National Museum, Smithsonian Institution by Dr. J. Piatek. > read more


La Primitiva

standby for la primitiva photo

Between 1888 and 1911, six iron masses weighing together 27.4 kg were found in and around the nitrate plants in the Tarapaca Region of Chile. The photo above shows a 74 g partial slice, acquired from Sergey Vasiliev—SV Meteorites by Dr. J. Piatek.


Bellsbank

standby for bellsbank photo

A 38 kg iron mass was found just below the surface in Cape Province, South Africa, in 1955. The photo above shows a 22.2 g partial slice, acquired from the United States National Museum, Smithsonian Institution by Dr. J. Piatek.


Twannberg

standby for twannberg photo

A 15.91 kg iron was found in May 1984 by M. Christen in a field on Twann Mt. (Twannberg), Switzerland, in association with glacial till transported by the Rhône glacier during the last ice age (Hofmann et al., 2009). A 2.2 kg paired mass was discovered by M. Jost in the attic of an old house in the village of Twann in January 2000. In 2005, a third mass was discovered as part of a rock collection in the Natural History Museum Bern, which previously belonged to the Schwab Museum, and which was incorrectly labeled as hematite. In 2007 three more paired masses were found in the Twannbach canyon, and more recently many other transported masses were recovered near Twannbach River and Gruebmatt. Beginning in 2015, numerous masses have been recovered at Mont Sujet plateau. These are non-transported masses in their original fall location, delimiting a strewn field of over 4.5 km in a direction ENE to WSW (Hofmann et al., 2016). These recent recoveries raise the TKW to ~70 kg in ~550 individual pieces (Smith et al., 2016). The present total recovered weight is in stark contrast to the estimated ~30 million kg mass of a 20-m-diameter meteoroid calculated to have fallen 165 (±58) t.y. ago (Smith et al., 2016). With the discovery of Twannberg and its observed genetic relationship with the three irons presented above, the iron grouplet became known as the Bellsbank Quartet. The photo above shows a 74 g slice from the first discovered mass, acquired from the Jim Schwade Collection by Dr. J. Piatek.


Guanaco

standby for guanaco photo

A single 13.1 kg iron meteorite was found in Antofagasta, Chile in 2000. Guanaco became the requisite fifth recognized member of this Ni-poor (4.3%), schreibersite-rich iron group (the Bellsbank Quintet), and therefore, John T. Wasson has proposed that this new group be given the designation IIG. Shown in the photo above is a 310 g slice acquired from Rodrigo Martinez—Atacama Desert Meteorites by Dr. J. Piatek. > read more


© 1997–2019 by David Weir

Posted on Leave a comment

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.


Posted on Leave a comment

Guanaco

Iron, IIG
compositionally linked to group IIAB
standby for guanaco photo
Found 2000
25° 06′ S., 69° 32′ W. A single iron mass weighing 13.1 kg was found by a geologist in El Guanaco, near Aguas Verde, in the Atacama Desert of Chile. The Guanaco mass was submitted for analysis to UCLA (J. Wasson), and elemental ratios were compared to those of the similar schreibersite-rich iron meteorite, La Primitiva, which was also found in the Atacama Desert in Chile. The two iron meteorites exhibit significant differences, notably, in the ratios of As, Au, and Ir, and it was determined that they represent separate falls. Minor fusion crust was reportedly found on the exposed surface of the Guanaco mass, attesting to a relatively recent fall.

Members of this iron group are the most phosphorus-rich meteorites known, with abundant low-Ni schreibersite ribbons present throughout. The 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).

Guanaco is the fifth member of the compositional grouplet that comprises the iron meteorites formerly known as the Bellsbank Quartet—Bellsbank, La Primitiva, Tombigbee River, and Twannberg (see photos below). Therefore, Guanaco provides the requisite number of members needed to establish a new iron chemical group—The Bellsbank Quintet. John T. Wasson has proposed that this new iron group be designated IIG, its members representing the most Ni-poor, P-rich meteorites of all iron groups. Guanaco has a Ni content of 4.79%, while that of other members of the group ranges from a low of 4.3% (Tombigbee River), to 4.5% (Bellsbank), to a high of 5.1% (La Primitiva, Twannberg).

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 IIG group (Bartoschewitz, 2003). This iron portion also has the lowest Ni content (3.5%) of any known iron meteorite. In addition, several pairings of Twannberg have been recovered in recent years, including a 2.246 kg mass which was discovered in January 2000 by M. Jost in the attic of an old house in the village of Twann, and a third mass weighing 2.533 kg discovered in 2005 as part of a rock collection in the Natural History Museum Bern. Besides these two large masses, three smaller fragments were found in 2007 in the Twannbach canyon by M. Wälti and D. Ducrest, bringing the TKW of Twannberg to ~20.771 kg. All six of the Twannberg masses are associated with glacial till transported by the Rhône glacier during the last ice age (Hofmann et al., 2009).

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 Guanaco mass has been terrestrially weathered, but remnant fusion crust and heat-affected zones still remain. Neumann bands record shock forces, including the violent entry into Earth’s atmosphere.

The 27 g partial slice of Guanaco shown above exhibits schreibersite ribbons and patches, while the top photo below shows the regmaglypted main mass as found. The middle photos below show the complete Twannberg stone and an interior view of the main mass, with its similarity to Guanaco; compare also to the bottom photo, a rare slice of La Primitiva.

standby for guanaco photo
Guanaco—photo courtesy of Atacama Desert Meteorites

standby for twannberg photo
twannberg
Twannberg—Top: complete stone photographed by Rolf Buehler; Bottom: sectioned main mass
click on the bottom photo for an enlarged view

standby for la primitiva photo
La Primitiva—a 74.2 g full slice of this IIG iron
Photo courtesy of Sergey Vasiliev—SV-meteorites