VERMILLION


Pyroxene Pallasite
Vermillion duo
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Found May, 1991, recognized 1995
39° 44.18' N., 96° 21.68' W.

A 34.36 kg mass was found by M. and G. Farrell while planting in a grain field in Marshall County, Kansas, in the vicinity of the Black Vermillion River. The iron meteorite was purchased by a dealer with the belief that it was a Brenham mass, but upon cutting, it was discovered to be a unique pallasitic iron. The first analyses and classification of Vermillion was conducted by Boesenberg et al. and published in Meteoritics, vol. 30, #5, p. 488–489 (1995). They determined that Vermillion was a pyroxene pallasite genetically related to the only other pyroxene pallasite known at the time, Yamato 8451.

Vermillion consists of ~86 vol% FeNi-metal and ~14 vol% silicates with grains much smaller than normal (Boesenberg et al., 1995). The silicates consist of olivine (~93 vol%), orthopyroxene (~5 vol%), chromite (~1.5 vol%), and merrillite (~0.5 vol%). The composition of Vermillion is comparable to Y-8451 (54.9 g), which has a very similar silicate composition consisting of olivine (~94 vol%), orthopyroxene (~4.8 vol%), clinopyroxene (~1.1 vol%), and merrillite (~0.1 vol%) as reported by Boesenberg et al. (1995) and Yanai and Kojima (1995). Pyroxene accounts for ~0.7 and ~1.6 vol% of the silicate fraction of Vermillion and Y-8451 respectively, the remainder being mostly olivine. By comparison, the main-group pallasites primarily contain olivine with only trace amounts of pyroxene. The coexistence of both olivine and pyroxene in these two pallasites might indicate a lower crystallization temperature. In light of the compositional and isotopic similarities between Vermillion and Y-8451, Boesenberg et al. (1995) proposed they be recognized as a new grouplet (duo).

Subsequent studies determined that Vermillion shares a similar pyroxene composition, mineralogy, O-isotope composition, and REE pattern not only with Y-8451, but also with the more recently discovered pyroxene pallasite Choteau (8,474 g). It was therefore proposed by Gregory et al. (2016) that these three pyroxene pallasites be recognized as members of a new pyroxene-pallasite grouplet termed 'Vermillion pallasites'. In addition, the grouplet of three Vermillion pallasites recognized by Gregory et al. (2016) have O-isotopic compositions similar to that of the ungrouped olivine pallasite Hassi el Biod 002 and plot in the field of the acapulcoite–lodranite 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.

Oxygen Isotope Composition of Ungrouped Pallasites
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Diagram adapted from Gregory et al., 47th LPSC, #2393 (2016)

However, significant differences that exist between these three Vermillion pallasites are not yet resolved, including differences in texture (Y-8451 contains 4× the vol% of silicates as Vermillion) and siderophile trace element composition, as well as the presence of the carbide cohenite in Vermillion. In addition, although Vermillion, Y-8451, and Choteau all contain both low- and high-Ca pyroxenes of similar compositions, some differences are evident. Vermillion and Y-8451 contain both pyroxene types in the form of large grains, inclusions in olivine, and grains bordering olivine. By contrast, high-Ca pyroxene in Choteau has only been identified as inclusions in olivine, while low-Ca pyroxene is present as both individual grains and along boundaries of high-Ca pyroxene grains (Gregory et al., 2016). Furthermore, plagioclase has not been found in either Vermillion or Y-8451, but is present in Choteau as inclusions in both low- and high-Ca pyroxene and as small veins in low-Ca pyroxene. Notably, plagioclase is also present in the ungrouped pyroxene pallasite NWA 10019, but it is compositionally distinct to that in Choteau.

The olivine fayalite composition of these pyroxene-bearing pallasites plots at the magnesian end of the main-group pallasite range. As a comparison, the Eagle Station pallasite group has the most ferroan composition, as well as a high Ge/Ga ratio in the metal and a unique O-isotope composition. The high Ir content in the Eagle Station pallasites suggests crystallization from the inner core region of its parent body, below the core–mantle interface in which the main-group and pyroxene-bearing meteorites probably formed on their respective parent bodies.

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 analysis. 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 the Choteau pallasite from the Vermillion pallasite (see diagram below). The new ε54Cr–Δ17O coupled diagram shows that Vermillion plots in a unique space between the winonaite and acapulcoite–lodranite fields, while Choteau plots in the ureilite field.

Oxygen and Chromium Isotope Systematics for Pallasites
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Diagram credit: Dey and Yin, 53rd LPSC, #2428 (2022)

In a previous study, Dey et al. (2019 #2977) made use of Δ17O 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 versus 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 (see diagram here). A similar comparative analysis of the metal (chromite as proxy) and silicate phases was performed for the Vermillion and Imilac pallasites, and two additional Eagle Station pallasite group members, Cold Bay and Itzawisis. The results are consistent with an endogenous mixing process for each of these pallasites (see diagram above). Other results from their study can be found on the Caddo County and Miles pages.

Siderophile trace element and oxygen isotopic compositions clearly resolve the pyroxene-bearing meteorites from the main-group and Eagle Station pallasites, and therefore they represent additional parent bodies on which pallasite-like textures were formed. In addition to Vermillion, Y-8451, and Choteau, several other pyroxene-bearing meteorites have subsequently been recognized. A 46 g pyroxene-rich pallasite named Zinder was found in Niger in 1999. It contains 28 vol% pyroxene and 27 vol% olivine in a network of FeNi-metal. In 2003, a 53 g pyroxene-rich pallasite designated NWA 1911 was purchased in northwest Africa. It has a modal composition of about 24% FeNi-metal and 75% silicates, with the silicates consisting of 34.5% orthopyroxene and 40.2% olivine. Zinder and NWA 1911 were grouped together by Boesenberg and Humayun (2019), Separate fragments of the ungrouped pyroxene-bearing pallasite NWA 10019, weighing together 606 g, were found in 2015.

On a Ni vs. Au coupled diagram, Vermillion plots just outside of the low-Au end of the IAB main group irons, and also plots along an extension of the low-Au, medium-Ni (sLM) subgroup into lower Au compositions (Wasson and Kallemeyn, 2002). Furthermore, the O-isotopic composition of Vermillion is within the range of IAB irons, and therefore it could be genetically related to members of the low-Au division of the IAB iron-meteorite complex.

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. Proposed scenarios for pallasite formation can be found on the Imilac page. The specimen of Vermillion pictured above is a 67.9 g partial slice. The photo below shows the etched reverse side exhibiting a fine octahedrite pattern.

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