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Auburn

standby for auburn photoIron, IIAB: Compositionally related to group IIG
Found: before 1867, Coordinates: 32 ° 37′ N., 85 ° 30′ W., approx.
Formerly considered a probable transported mass of Tombigbee River (IIG)

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, named Tombigbee River, had a combined weight of 43.8 kg. In 1867, an extensively oxidized 3.63 kg mass was plowed up on the Daniel Plantation located ~250 km east of the Tombigbee River find and ~1 mile west of East Alabama College in Auburn. The severely weathered mass of Auburn was broken up with a sledge hammer in a blacksmith’s shop and possibly artificially heated before being described by several different researchers (V. Buchwald, 1975). Although Auburn has historically been considered to be a transported piece of Tombigbee River, it is now demonstrated by Hilton and Walker (2019) to more likely represent a separate iron belonging to the IIAB group. The classification of the Auburn iron meteorite has a long and varied history as described in the following entry from Grady’s Catalogue:


‘A mass of about 8lb (3.63kg) was ploughed up near East Alabama College, C.U. Shephard (1869). Described, with an analysis by O. Hildebrand, 4.67 %Ni, E. Cohen (1905). A second analysis, by A.A. Moss, gave 5.9 %Ni. Classification and analysis, E.R.D. Scott et al. (1973). Description, V.F. Buchwald (1975). Auburn has had a turbulent history, first recognised as an individual meteorite, then demoted to a transported piece of Tombigbee River ( q.v._ ). Correspondence between R.S. Clarke Jr., V. Buchwald and J.T. Wasson in 1994 (copies in Min. Dept., NHM, London) has prompted the re-instatement of a Catalogue entry for Auburn. V.F. Buchwald, pers._ commun._ (1994), notes that the octahedral structure and absence of schreibersite differentiates Auburn from Tombigbee River, but J.T. Wasson, pers._ commun._ (1994), takes the view that Auburn is a fragment from Tombigbee River. Until the relationship, or otherwise, between Auburn and Tombigbee River is established beyond doubt, then there is more to be gained than lost by keeping separate entries for the two specimens.’


Wasson and Choe (2009) argued that group IIG irons are chemically 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).

Hilton and Walker (2019) conducted a chemical and isotopic study of each of the IIG irons including a sample of the Auburn mass. They determined that Auburn has a significantly higher Ir concentration than all members of the IIG group, but it is consistent with some IIAB irons. In addition, they demonstrated that IIG and IIAB irons have similarities with respect to their Re–Os isotopic systematics. Furthermore, they found that Auburn has HSE abundances that are different from the IIG irons, but are consistent with some IIAB members such as Coahuila. These data suggest a likely genetic relationship between Auburn and the IIAB group irons, and they plan to use Mo isotopes in future studies to determine whether or not a genetic connection exists between the IIG and IIAB group irons (see diagrams below).

standby for auburn chemical and isotopic diagrams
Diagrams credit: Hilton and Walker, 50th LPSC, #1240 (2019) – see abstract text for a full explanation of the diagrams

The 13.3 g angular specimen pictured above is a corroded fragment with label provenance from the Auburn mass, previously part of the Thomas M. Bee Collection. Most specimens of the Auburn meteorite are similar in size to this one or smaller, consisting of small angular fingers of kamacite after undergoing significant terrestrial corrosion and disintegration over the intervening years. The photo below shows two large fragments of Auburn weighing 1.7 and 0.7 kg

Image Credit: Vagn F. Buchwald, Handbook of Iron Meteorites Volume 2 University of California Press, p. 276 (1975)

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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

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Udei Station

Iron, IAB complex, Udei Station grouplet
standby for udei station photo
Fell spring 1927
7° 57′ N., 8° 5′ E. A 103 kg mass was seen and heard to fall in the daytime near the Benue River in Nigeria. The Geological Survey of Nigeria learned of the event in 1935 when the mass was recovered from a shallow hole six miles west of the railway station at Udei and 23 miles north of Makurdi. Some reports state that a second mass exists.

The IAB iron-meteorite complex, recently proposed by Wasson and Kallemeyn (2002), comprises iron meteorites from the former IAB-IIICD group, as well as numerous related irons. Many of the members contain silicate inclusions with roughly chondritic compositions. Udei Station is a low-Au member of the IAB complex that is closely related to the main group. On a Ni–Au diagram, a grouplet of six members has been resolved in an area close to the sLL subgroup, but with anomalous Ni contents; these meteorites were designated the Udei Station grouplet. Although theories of formation for these irons commonly attribute their origin to impact-melt pools, correlations between Ni abundance and several chemical and physical properties alternatively suggest an interaction with a crystallizing metallic core. The absolute I–Xe retention age, relative to the Shallowater standard, was calculated to be ~4.558 b.y., while the metamorphic resetting of the K–Ar chronometer occurred ~4.31 b.y. ago (Bogard et al, 2005; Pravdivtseva et al, 2013).

Udei Station is a medium octahedrite that contains a high proportion of silicates and troilite. Silicate inclusions range in size from single grains to as large as 8 cm, and they are composed of enstatite, olivine, oligoclase, and diopside representing various subchondritic rock types including basalt–gabbro, feldspathic orthopyroxenite, lherzolite, and harzburgite. Some of the inclusions crystallized from a low degree partial melt (~3–10%) reaching a peak temperature of <1180°C, while others represent a partial melt residue (Ruzicka and Hutson, 2009). Identification of an FeO-poor, Na-rich gabbroic inclusion similar to one found in Caddo County is only the second discovery of such material. It was concluded that interstitial silicate melt migration and removal occurred, assisted by CO gas (now manifest as graphite), while low abundances of metallic melt (~37%) became trapped in veins between solid silicate assemblages (Ruzicka and Hutson, 2009). One plausible model involves impact processing within small melt pools that led to brecciation and mixing of rock clasts from different locations having different thermal histories (ranging from igneous to chondritic) with metallic melt. These melt pools were then slowly cooled, consistent with burial within a deep regolith.

A formation model that accounts for such a wide variety of inclusions and disparate petrographic evidence was proposed by Ruzicka and Hutson (2009) and Ruzicka (2014). They conceive of a major collisional disruption/jumbling of a partially molten, incompletely differentiated planetesimal. They consider winonaites to be derived from the outer layers of this body. At the same time, they envision the IAB complex irons that were more rapidly cooled, exhibit older I–Xe ages, and which are Ni-rich, to have derived from the middle layers, while those that were more slowly cooled, exhibit younger I–Xe ages, and which are Ni-poor, to have derived from the lowest layers. This was a severe impact that mixed material from different depths on the parent body—solid subchondritic material from the cooler regions, and fractionated material from very hot regions—causing metallic melt to be injected into the silicates.

Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] and the ungrouped Caddo County [Udei Station grouplet] and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIs. Based on the constraints provided by the timing of metal segregation, they modeled the early history of the 120(+)-km-diameter IAB parent body as outlined in the following diagram: standby for IAB formation history diagram
Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open access link)
‘Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling’
(https://doi.org/10.1016/j.epsl.2017.11.034)’
Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages. 17O vs. ε54Cr for Irons and Pallasites
standby for o-cr isotope diagram
click on photo for a magnified view

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
Udei Station has a Cl–Ar-based CRE age of 123 m.y. (Albrecht et al., 2000). It plots as an anomalous member of group IAB in several regards. It contains a clast best described as a peridotite, composed of the most Fe-rich mafic minerals found among IAB irons. Some variability in appearance is apparent in some members of this grouplet, as can be seen in the Four Corners meteorite. The specimen shown above is a 26.3 g partial slice exhibiting a high content of silicates.


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Toluca

Iron, IAB complex, sLL subgroup
standby for toluca photo
Found 1776, known earlier
19° 34′ N., 99° 34′ W. Many large masses were found near Xiquipilco, Mexico, the largest of which weighed 300 pounds. Recently, a taxonomic revision was proposed by Wasson and Kallemeyn (2002) that includes iron meteorites from the IAB-IIICD group, along with numerous IAB-related meteorites. On a Ni–Au diagram, Toluca and other similar irons compose a low-Au, low-Ni subgroup (sLL).

The I–Xe closure age of Toluca was determined by Pravdivtseva et al. (2009) for both high-Mg and low-Mg pyroxenes, the two closure times differing by 8.5 (±4.4) m.y. An absolute closure age based on the Shallowater standard was calculated to be 4.5605 (±0.0024) b.y. for high-Mg pyroxenes and 4.5520 (±0.0037) b.y. for low-Mg pyroxenes, a range similar to that of other IAB iron silicates. The earlier closure age may be commensurate with a catastrophic disruption of the IAB parent body. The research team also determined the cooling rate of Toluca following parent body breakup and reassembly. This was calculated as a function of the difference between the crystallization temperatures and closure ages of the two pyroxenes to be 14.5 (±10.0)°C/m.y.

A comparative study of the IAB iron main group (MG) and sLL subgroup by Worsham et al. (2013, 2016, 2017) demonstrated through Mo isotope compositions that both groups derive from a common parent body that was initially chondritic. They verified through HSE data that irons from these two groups crystallized from distinct parental melt pools, with the smaller sLL formation event occurring 0–3 m.y. after the MG formation event. Moreover, the observed fractionations are not the result of fractional crystallization, but instead, most likely involved crystal segregation and other processes related to the respective impacts. Further evidence for formation in distinct melt pools among the IAB iron groups was found through cooling rate studies correlating the cloudy zone particle size with the metallographic cooling rate (Goldstein et al., 2013). Moreover, the slow cooling rates determined for the IAB irons and other meteorite groups containing silicate assemblages (e.g., pallasites and mesosiderites), were found to be inconsistent with the faster cooling rates attributed to those iron groups that underwent fractional crystallization in cores lacking insulating silicate mantles (e.g., IIIAB, IVA, and IVB).

Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] and the ungrouped Caddo County [Udei Station grouplet] and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIs. They contend that a catastrophic breakup and reassembly occurred during which different silicate lithologies were mixed. Based on the CRE-corrected W data, Worsham et al. (2017) derived a segregation age corresponding to 3.4 (±0.7) and 5.0 (±1.0) m.y. after CAIs for the MG and sLL subgroup, respectively. They argue that a breakup and reassembly event would have also mixed different metal lithologies together, and would have equilibrated the W systematics of the MG and sLL subgroup. The top schematic diagram below is the model of Hunt et al. (2018), which shows the early history of the 120(+)-km-diameter IAB parent body based on constraints provided by the timing of metal segregation. The schematic diagram beneath that one shows the impact-generated melt model of Worsham et al. (2017). standby for IAB formation history diagram
Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open access link)
‘Late metalï ¿ ½silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling’
(https://doi.org/10.1016/j.epsl.2017.11.034)

standby for iab iron formation diagram
Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 164 (2017)
‘Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex’
(https://doi.org/10.1016/j.epsl.2017.02.044)
Based on the similar silicate textures, reduced mineral chemistry, and O and Mo isotopes, it is presumed that the winonaites and the IAB complex irons originated on a common parent body. Utilizing a Ge/Ni vs. Au/Ni coupled diagram, Hidaka et al. (2015) determined that FeNi-metal in the winonaite Y-8005 plots in the field of the sLL subgroup of the IAB complex irons. Worsham et al. (2017) also demonstrated that the Mo isotope data for the two winonaites they studied, Winona and HaH 193, attest to a common parent body for winonaites and MG/sLL irons. Moreover, the metal in Y-8005 retains a near chondritic composition likely representative of the precursor material of the parent body. In view of these findings, Hidaka et al. (2015) suggest that the sLL subgroup rather than the MG represents the primitive metal of the IAB–winonaite parent body, with the MG possibly representing a partial melt of the sLL subgroup.

Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages. 17O vs. ε54Cr for Irons and Pallasites
standby for o-cr isotope diagram
click on photo for a magnified view

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
Toluca has a high CRE age of 600 (±150) m.y. (Chang and Wänke, 1969). The silicate ureyite (NaCrSi2O6) has been found as a rare occurrence in Toluca (Frondel and Klein, 1965). Further information on the formation of the IAB iron complex can be found in the Appendix, Part III. The specimen of Toluca shown above and below is a 2003 Harvey Award—’New Technology Award’—which was presented in recognition of the Meteorite Studies website. standby for toluca photo