CHOTEAU

Metal-rich Lodranite or Pallasitic Lodranite
(Pallasite-ungrouped in MetBull 102)
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Purchased 2011
no coordinates recorded

A slightly weathered 8,474 g metallic mass showing a few protruding olivine crystals was discovered at an estate sale in Choteau, Montana by Debbie Cilz of the Montana Meteorite Lab. Analysis and classification was conducted at Washington University in Seattle (Irving and Kuehner), and the meteorite was determined to be an ungrouped pallasite.

Photo of the exterior of a 303 g end section of Choteau
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Photo credit: Ruben Garcia in 'Meteorite Picture of the Day' for June 20, 2013.

Choteau is composed of FeNi-metal and angular olivine in a ratio of ~60:40, along with accessory phases that include pyrrhotite, schreibersite, merrillite, chromite, and orthopyroxene. Symplectitic intergrowths consisting of chromite+orthopyroxene occur along some grain boundaries. An oxygen isotope analysis of Choteau was conducted at the University of New Mexico (K. Ziegler) and the values were found to be consistent with those of the acapulcoite-lodranite clan (MetBull 102 diagram), and nearly identical to the lodranite NWA 6685 (MetBull 100 diagram).

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Previously, the compositional and isotopic similarities between the Vermillion and Y-8451 pallasites led Boesenberg et al. (1995) to propose that they be recognized as a new grouplet (duo). Subsequent studies by Gregory et al. (2016) showed that Vermillion shares a similar pyroxene composition, O-isotope composition, and REE pattern not only with Y-8451, but also with the pyroxene pallasite Choteau. Therefore, they proposed that these three pyroxene pallasites be recognized as members of a new pyroxene-pallasite grouplet termed 'Vermillion pallasites'. Better resolution of the pyroxene pallasites, as well as others belonging to the main-group pallasites, was obtained by Dey and Yin (2022) through the use of Cr isotope analyses. They discovered that although the three pyroxene pallasites Vermillion, Y-8451, and Choteau could be grouped together based on oxygen isotopes and other mineralogical similarities, nucleosynthetic 54Cr isotopes clearly resolve Choteau from Vermillion (see diagram below). The new coupled ε54Cr–Δ17O diagram shows that Vermillion plots in a unique space between the fields for winonaites and the acapulcoite–lodranite clan, while Choteau plots in the ureilite field and extends the field of the acapulcoite–lodranite clan to lower ε54Cr values.

ε54Cr vs. Δ17O Diagram for Meteorite Groups
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Diagram credit: Vaci et al., Nature Communications, vol. 12, #5443 (2021, open access link)
Adapted by Dey and Yin, 53rd LPSC, #2428 (2022)
'Diversity Of Pallasites In The Early Solar System'

On the oxygen isotope diagram below, Irving et al. (2022) demonstrate that Choteau, together with the lodranite NWA 6685, plot precisely along the least-squares regression line originally established by Clayton and Mayeda (1996) from data points for the acapulcoite–lodranite clan, and which has been further populated by many accepted and proposed (e.g., Erg Atouila 001, NWA 468) related meteorites. If Choteau is confirmed to be genetically related to the acapulcoite–lodranite clan, it may be difficult to decide which terminology is more useful in describing this meteorite: that used by Irving et al. (2022) who identify it as a metal-rich lodranite, or alternatively, an identification as a pallasitic lodranite in keeping with its historical recommendation. This issue was examined by Boesenberg et al. (2000) with respect to the pyroxene pallasites Vermillion and Yamato 8451.

Oxygen Isotope Plot For Erg Atouila 001 and Acapulcoite–Lodranite Clan Meteorites
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Diagram credit: Irving et al., 53rd LPSC, #2059 (2022)
'Ultrasodic, Albite-Rich Syenitic Achondrite Erg Atouila 001: A Highly Evolved
Felsic Igneous Differentiate From The Acapulcoite-Lodranite Parent Body'

Hilton and Walker (2019) conducted a chemical and isotopic study of each of the IIG irons including a sample of the Auburn mass. They realized 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).

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

A common metal melt origin for IIAB and IIG irons during core crystallization is supported by chemical–AU trends consistent with fractional crystallization (Wasson and Choe, 2009). A genetic connection is further supported by Cr and O isotope data newly obtained by Anand et al. (2022 #1891). On an ε54Cr vs. Δ17O coupled diagram, these irons plot in both the ureilite field and the acapulcoite–lodranite field extended to lower ε54Cr values. Based on metallographic cooling rate data, the IIABG group parent body diameter was inferred to be ~330 km (200–460 km; Kaminski et al., 2020 and references therein), which is significantly smaller than that calculated for the ureilite parent body. The ε54Cr and Δ17O values for IIAB Sikhote Alin (chromite), IIG Twannberg (troilite), and Choteau (olivine) are identical within error. Although the two O–Cr isotope diagrams below are shown at different scales, together they depict that Choteau and IIABG irons plot very close to each other in ε54Cr vs. Δ17O space; while this may be merely coincidental, it could reflect the existence of a genetic relationship. It is interesting that, as noted by Schrader et al. (2022 #6132), both the silicate compositions of acapulcoites are similar to those of IIAB irons (see McCoy et al., 1996, GCA, vol. 60, #14, Table 1. p. 2684; Schrader et al., 2017, GCA, vol. 205, p. 11) and the oxygen fugacity (ƒO2) of acapulcoites is similar to IIAB irons (see Schrader et al., 2017, GCA, vol. 205, p. 12, 23).

ε54Cr vs. Δ17O Diagram for IIABG Irons and Choteau
note: the verticle blue bar in the left diagram represents the ε54Cr variation in IIAB irons
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Diagrams credit:
left: Anand et al., 53rd LPSC, #1891 (2022)
right: Dey and Yin, 53rd LPSC, #2428 (2022)

A beautiful etched slice of Choteau is shown in 'Meteorite Picture of the Day' for June 20, 2013, courtesy of Ruben Garcia. The 24.97 g slice of Choteau pictured above was photographed by Ashley Rose Humphries.