Classification: Carbonaceous Chondrite (CM-anomalous)
Explainer: The CM-an classification means that the sample is a carbonaceous chondrite (C) which belongs to the Mighei (M) group of meteorites. Mighei, was the first meteorite of this type which fell in Ukraine in 1889 (hyperlink https://www.lpi.usra.edu/meteor/metbull.php?code=16634 ). The CM group of meteorites are the most common type of carbonaceous chondrites. They are often rich in hydrated minerals and organic mineral phases, along with some minor silicate mineral components. Typically they exhibit a range of different alteration states due to interactions with fluids on their parent asteroids. In this particular case, sample OUT 18012 does not fit into the usual range of petrological types, as it lacks key hydrated clay minerals that we would usually expect to see – this makes it anomalous (an) which is why we call the sample a CM-an. CM chondrites like this one and other examples likely formed on parent asteroids outside of the orbit of Jupiter, before they were transferred into the main asteroid belt during the early stages of Solar System body migration. You can see more about where this sample fits into the meteorite classification scheme by looking at this page.
Description: Taken from the Meteoritical Bulletin for OUT 18012
History: The meteorite was recovered as part of the Lost Meteorites of Antarctica project, which was funded in the UK by the Leverhulme Trust and supported by the British Antarctic Survey and the University of Manchester. These samples were collected as part of the project’s first field season in austral summer December 2018 – January 2019 by a two person field party consisting of Katherine Joy and Julie Baum. Found on blue ice surface. Altitude 1410 m.
Physical characteristics: Mass: 13.99 g. Pieces: 1. Dimensions: 4 × 3 × 3 cm. A round, whole stone with 5-10% black fusion crust and brown green exposed interior with abundant white inclusions. After cutting, the freshly exposed interior is dark gray to black.
Petrography: Thin section shows mostly small chondrules <0.5 mm in diameter, abundant isolated olivine and pyroxene grains and a few CAIs (up to 0.1 mm) set in a fine-grained Fe-rich matrix which has a smooth appearance in back-scattered electron images; minor small sulfide grains are present. Chondrules are mostly porphyritic and FeO-poor; FeO-rich chondrules with zoned olivine are less common; there are rare barred olivine chondrules. Chondrule mesostasis is altered. Chondrules and mineral fragments are often surrounded by fine-grained rims. Abundant calcite up to 0.3 mm is distributed throughout the section. (A. J. King, J. Najorka, NHM London) X-ray diffraction (XRD) of two polished thin sections indicates that the main phases in the matrix are olivine and pyroxene; hydrated phases (e.g. serpentine, tochilinite) that are typically found in type 2 chondrites were not detected.
Mineral compositions and geochemistry: All analyses by EPMA. Olivine shows a wide compositional range from Fa0.6 to Fa37.0 (N=13), with CaO up to 0.5 wt%, Cr2O3 up to 0.5 wt% and Al2O3 up to 0.3 wt%. Olivine histogram has a peak at Fa0.8: Fa0.8±0.3 (N=7), Fa26.5±10.5 (N=6). Pyroxene En94.9±3.0Fs3.0±2.3Wo 2.1±1.5 (N=12) and En52.2±5.0Fs1.4±0.2Wo 46.4±5.2 (N=2). Analysis of the matrix (~10 µm spot size) resulted in analytical totals of ~91 wt% (SiO2 26.2±1.0 wt%, FeO 40.3±2.6 wt%. MgO 13.1±1.4 wt%, N=10). Oxygen Isotopes (R. Findlay, J. Malley and R. Greenwood, OU): One fragment was analyzed for oxygen isotopes by laser fluorination at OU. A ~36 mg homogenized sample yielded the following results (per mil deviation from VSMOW): δ17O=1.087, δ18O=7.984, Δ17O=-3.065. Δ17O values are calculated using a slope of 0.52. The oxygen isotope analysis of the fragment plots within the CM2 field.
Specimens: 11.321 g type specimen (main mass) held at the NHM London.
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