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 elementAu 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 redistribution 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).
A common metal melt origin for IIAB and IIG irons during core crystallization is supported by chemicalAU 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 in the acapulcoitelodranite field extended to lower ε54Cr values. Based on metallographic cooling rate data, the IIABG group parent body diameter was inferred to be ~330 km (200460 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 OCr 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, possibly reflecting some form of a genetic relationship between them all. 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 are similar to those of 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
left: Anand et al., 53rd LPSC, #1891 (2022)
right: Dey and Yin, 53rd LPSC, #2428 (2022)
Guanaco is the fifth member of the compositional grouplet that comprises the iron meteorites formerly known as the Bellsbank QuartetBellsbank, La Primitiva, Tombigbee River, and Twannberg (see photos below). Therefore, Guanaco provides the requisite number of members needed to establish a new iron chemical groupThe 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 (HOgg). 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.
Guanaco complete mass
Photo courtesy of Atacama Desert Meteorites
TwannbergTop: complete stone photographed by Rolf Buehler; Bottom: sectioned main mass
click on photo for an enlarged view
La Primitiva74.2 g full slice of this IIG iron