Eleven conjoint fragments with a combined weight of 321 g were found in Western Sahara and subsequently purchased by J. Sinclair and J. D. Cline at the 2019 Tucson Gem and Mineral Show. A type sample was sent to the Appalachian State University in Boone, North Carolina (A. Love) for analysis and classification, and NWA 12860 was determined to be a rare group C3 mesosiderite.
The meteorite is a breccia with a modal composition of 26% FeNi-metal, ~58% orthopyroxene, 4% plagioclase, and 1% olivine, along with accessory troilite, chromite, and apatite. Based on the silicate abundances of 93 vol% orthopyroxene and 6 vol% plagioclase, this mesosiderite is consistent with class C. In addition, the recrystallized textures indicate metamorphic stage 3. The meteorite exhibits a low degree of shock and terrestrial weathering.
Based on the metamorphic textures of the matrix silicates, a scheme was developed (Powell, 1971; Floran, 1978) which assigned the mesosiderite group members into one of four textural categories; 1) minimally recrystallized, 2) moderately recrystallized, 3) highly recrystallized, or 4) intergranular melt rock. However, clear differences in bulk composition among these four categories prompted a reinterpretation of this scheme (Hewins, 1984). Additional discrimination criteria for primitiveness were investigated by Sugiura et al. (2013). They determined that NWA 1878 was more primitive than other mesosiderites in category 1, so it was designated the first metamorphic type 0.
Hewins proposed a further division for the least metamorphosed, at that time category 1, based on plagioclase abundance: a higher abundance for group A1 (24%) compared to a lower abundance for group B1 (21%). A further division of the more highly metamorphosed categories 2 and 3 was based on whether plagioclase or orthopyroxene matrix predominates (groups A2/A3 and B2/B3, respectively). The more basaltic, plagioclase-rich members of class A are enriched in an anorthitic, cumulate eucrite-like component, while the more ultramafic, orthopyroxene-rich members of class B are enriched in a diogenite-like component. The more plagioclase-rich compositional class A contains a larger diopside component and has a lower Mg# than the orthopyroxene-rich compositional class B.
Through other studies it was determined that the Ir/Ni ratios (or better still, a plot of Ir/Ni vs. Au/Ni) for matrix metal of mesosiderites is diagnostic for membership in group A or B, reflecting values of 0.000036 or 0.000051, respectively (Wasson and Rubin, 1985). According to Kong et al. (2008), group B may have assimilated a higher proportion of solidified, weakly fractionated (higher Ir, lower Ni and Au) metal compared to group A. Furthermore, the concentrations of Ga and Ge are lower in the metal of category 1 mesosiderites than in metal of more highly metamorphosed mesosiderites (Wasson et al., 1974). This is believed to have occurred as a result of reduction from silicates to metal during metamorphism.
Hewins reinterpreted the metamorphic orthopyroxene-rich groups B2 and B3 as having some melt-rock textures and assigned them to a new igneous group B4, reassigning the previous members of group 4 to A4. However, this reinterpretation has left groups B2 and B3 unrepresented. More recently, Hewins established a group C2 to accommodate the granular texture and very low plagioclase content (05%) of certain paired Antarctic orthopyroxenitic mesosiderites. However, the subsequent identification of igneous clasts in these mesosiderites led to their reassignment to group B4. For a more in-depth treatment, see R. Hewins, Meteoritics, vol. 23 (1988).
Wang and Hsu (2019) used PbPb chronometry to date 53 merrillite crystals associated with FeNi-metal in the Youxi mesosiderite. Based on the low REE abundances in the Youxi merrillite compared to that in eucrites, they contend that it was formed by oxidation of P in metal during the metalsilicate mixing event rather than during magmatic activity. They derived an age of 3.950 (±0.080) b.y. which they consider represents the timing of merrillite development during the mesosiderite-forming event. An equally plausible timing for the metalsilicate mixing event was ascertained by Haba et al. (2019) using high-precision UPb dating of zircons in several mesosiderites. Based on these results they contend that the metalsilicate mixing event occurred 4.52539 (±0.00085) b.y. ago. They propose a scenario in which a hit-and-run collision disrupted the northern hemisphere of Vesta leading to ejecta debris reaccreting to the opposite, southern hemisphere (see schematic diagram below). The deeply buried mesosiderite meteorites were ejected into Earth-crossing orbits by later impacts.
Diagram credit: Haba et al., Nature Geoscience, vol. 12, #2, p. 512, (2019)
'Mesosiderite formation on asteroid 4 Vesta by a hit-and-run collision'
Although more than 250 mesosiderites are listed in the Meteoritical Bulletin Database, most have not been subjected to in-depth analyses in order to establish a specific geochemical group and metamorphic type. Notably, NWA 12860 is the second mesosiderite to be classified as group C3 after NWA 12566 (2,900 g) was approved in April 2019. The main masses of NWA 12860 are curated at the Pisgah Astronomical Research Institute in North Carolina. The photo of NWA 12860 shown above is a 3.18 g slice. The top photo below shows one of the larger fragments of NWA 12860, while the bottom photo shows an excellent petrographic thin section micrograph; both are presented courtesy of Anthony Love.
NWA 12860 105.64 g Fragment
click on photo for a magnified view
Thin Section Overview Photo in Plain Light
click on photo for a magnified view
Photos courtesy of Anthony LoveAppalachian State University, Dept. of Geological and Environmental Sciences