Pyroxene pallasite, ungrouped
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Found 18 Jan 2018
24° 40' S., 69° 45' W.
A relatively fresh meteorite weighing 4.88 kg was found by Luc Labenne within the Los Vientos (LoV) dense accumulation area in Antofagasta, Chile. The following is Luc's account of his find:
"I was in this fabulous mineral desert for meteorite hunting in January 2018 for the purpose of shooting a documentary for TF1 (TV show in Europe) when I picked up a very strange heavy stone, which did not have the obvious appearance of most meteorites. This stone in hand, I quickly understood, in the minute that followed the discovery, as being a very unusual meteorite and potentially a pallasite with small crystals."
A type sample was submitted for analysis and classification to several institutions, including the Museum National d'Histoire Naturelle (R. Hewins, S. Pont, and B. Zanda), the European Centre for Research and Teaching in Environmental Geosciences (J. Gattacceca and C. Sonzogni), and Florida State University (M. Humayun). Los Vientos (LoV) 263 was determined to be an ungrouped pallasite consisting of metal, olivine, and orthopyroxene in approximately equal proportions. The silicates contain small inclusions of troilite and chromite.
To date, seven pyroxene-bearing meteorites having a pallasite-like composition have been characterized: Choteau, LoV 263, NWA 1911, NWA 10019, Vermillion, Yamato 8451, and Zinder. Vermillion is composed of 86 vol% FeNi-metal and 14 vol% silicates, with the silicates consisting of 93% olivine and 5% pyroxene (4.9% opx and 0.1% cpx)equivalent to a modal composition of ~0.7 vol% pyroxene. The 54.8 g Y-8451 pallasite contains 57 vol% silicates consisting of 97% olivine, 2% orthopyroxene, 0.4% clinopyroxene, and 0.4% augite. The silicates in Y-8451 are modally equivalent to ~1.6 vol% pyroxene (Boesenberg et al., 2000). The 46 g Zinder pallasite has a high modal abundance of pyroxene, similar to that in NWA 1911, estimated to be 28 vol% (Wittke and Bunch, 2003). The modal abundance of silicates in NWA 10019 is ~60%, comprised of olivine (~4351 vol%) and orthopyroxene (~917 vol%) with pyroxene accounting for ~15 vol% of this pallasite (Boesenberg et al., 2016). The silicates in the LoV 263 pallasite are comprised of approximately equal proportions of olivine and orthopyroxene.
The metal component in LoV 263 has a similar siderophile element pattern to that of the Eagle Station pallasite, but the differences that do exist require formation under distinct redox conditions. In addition, LoV 263 metal has a low Ni content of 5.6%, and the meteorite plots in a distinct space on both Ga vs. Ni and Ga vs. Ge coupled diagrams (see diagrams below).
Ga vs. Ni for Irons and Pallasites
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Ga vs. Ge for Irons and Pallasites
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Diagrams credit: Sharma et al., 50th LPSC, #1442 (2019)
In a study conducted by Gregory et al. (2016), it was ascertained that Choteau is similar to both Vermillion and Y-8451 in mineralogy, composition, and with respect to oxygen isotopes, and they concluded that these three pyroxene pallasites form a grouplet; it was suggested that these three meteorites be termed 'Vermillion pallasites'. However, better resolution of many 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 analysis. They discovered that nucleosynthetic 54Cr isotopes clearly resolve the Choteau pallasite from the Vermillion pallasite (see the Vermillion page for additional details). Notably, on a coupled Δ17Oε54Cr diagram Choteau plots in the ureilite field. Other investigations of pyroxene pallasites found that the low-Ca pyroxene in Zinder, NWA 1911, NWA 10019, and LoV 263 is composed entirely of orthopyroxene (orthopyroxene in NWA 10019 contains ~100µm-sized clinopyroxene inclusions; Boesenberg et al., 2016), while that in Vermillion, Y-8451, and Choteau comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013; Sharma et al., 2019). Zinder contains a higher abundance of chromite compared to Vermillion, Y-8451, and Choteau.
Many of the pyroxene-bearing pallasites are associated with a number of established O-isotopic trends: Vermillion, Y-8451, and Choteau have O-isotopic compositions similar to that of the ungrouped olivine pallasite Hassi el Biod 002 and plot in the field of the acapulcoitelodranite clan on an oxygen three-isotope diagram (see plot 1 and 2); however, it was determined they are not related to that primitive achondrite group. Both NWA 1911 and NWA 10019 plot on the eucrite/mesosiderite fractionation line, which remains incompletely resolved from the bimodal fractionation trend of the main-group pallasites (Ziegler and Young, 2011; K. Ziegler, 2015). Although Zinder was demonstrated to be associated with NWA 1911 (Boesenberg and Humayun, 2019), it plots on the terrestrial fractionation line due to a difference in δ17O values; however, terrestrial weathering may be the reason for this difference. The O-isotope values for silicates in LoV 263 plot in a space distinct from all other known pallasites (see diagram below).
Oxygen Three-Isotope Diagram for LoV 263
Diagram from MetBull 108
Based on all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least ten separate parent bodies: (1) main-group high-Δ17O; (2) main-group low-Δ17O; (3) Eagle Station group; (4) Milton; (5) Vermillion + Y-8451; (6) Zinder + NWA 1911; (7) Choteau; (8) NWA 10019 ± Bordj Badji Mokhtar 001; (9) LoV 263; (10) Hassi el Biod 002. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. The specimen of LoV 263 shown above is a 24.33 g partial slice. The top photos below show the complete mass as found, while the bottom photo shows a 97.7 g full slice from this unique pallasite, courtesy of Luc Labenne.
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Photos courtesy of Luc LabenneMeteorites.tv