NWA 10155 lodranite achondrite meteorite

The NWA 10155 lodranite, a metal-rich example of achondrite meteorite

--- North West Africa.

Figure 1. Two modest slices of the lodranite NWA 10155. Source: Blaine Reed. The larger piece is 32x22x2 mm in size (5.99 grams), the smaller is 20x13x1.5 mm (1.65 grams). Note the abundant scattered grains of Ni-Fe metal (kamacite), with minor sulphide (troilite), and the blue-grey fractures lined by iron oxyhydroxides, presumably formed in terrestrial weathering. The larger slice has an uncorrected magnetic susceptibility of 13.7x10^-3 SI units (mean of 5 similar values measured with ZH Instruments SM-30 meter with 50-mm coil), equivalent to a log₁₀χ value of 4.98 (using assumed specific gravity of 3.5).

"Rock of the Month #238, posted for April 2021" ---

LODRANITES are a rare class of achondrite,

though the number of known representatives is growing steadily, due in no small part due to recoveries in the hot deserts of northwest Africa (the NWA series of finds). According to the Meteoritical Bulletin, accessed on 19 March 2021, there are 86 verified lodranites, of which 62 are NWA stones. The kindred acapulcoites have 81 members (37 NWA stones), while 11 more examples are of uncertain affinity, "acapulcoite-lodranites" (with 4 NWA stones), total for the two classes 178 meteorites. The numbers have increased rapidly in little more than three years (cf. note on NWA 11129, linked below). The total of verified meteorites in Meteoritical Bulletin stood at 65,253 as of 13 March 2021, such that acapulcoites and lodranites combined are just 0.27% of the total. In each class, the first representative was a fall, and a relatively large one at that. The first was Lodran, which fell in the Punjab district of what is today Pakistan, in 1868, weighing in at a healthy 1,000 grams. The first acapulcoite, more than a century later, was - surprise (!) - Acapulco, which fell in Guerrero state, southwest Mexico, in 1976, a solid 1,914 grams. The majority of these achondrites are quite small. 18 weigh 1 kg or more, the record-holder for size in each class being acapulcoite NWA 2656 at 7.5 kg and lodranite NWA 10265 at 3.36 kg (factors such as a low metal content and higher Fe content in olivine preclude NWA 10265 and our NWA 10155 being paired, that is, from the same meteorite shower). Lodranites are well-studied, with 121 records in the MINLIB bibliography, 1916 onwards, as of March 2021 (the term lodranite was coined in or by 1974).

NWA 10155

was found before February 2015, and purchased from a Moroccan dealer by Blaine Reed near the end of the 2015 Tucson show. The following summary is derived from the Meteoritical Bulletin record. The meteorite is small (total known mass 277 grams), a single individual of crystalline nature. Classification by C. Agee on a polished mount revealed polygonal pyroxene and olivine with common triple junctions on grain boundaries. Average grain size is circa 700 microns. Metal and sulphide are some 10% of the meteorite, which also contains accessory chromite and schreibersite. Olivine, orthopyroxene and clinopyroxene (diopside) were analysed. The olivine is magnesian, predominantly forsterite (Fa_4.9).

One can compare NWA 10155 with another, quite distinct lodranite, NWA 11129, which contains bright green chrome diopside, a distinctive pyroxene, but just 1-2% Ni-Fe metal. The olivine in NWA 11129 contains more Fe (Fa_11.4) compared to NWA 10155, which is consistent with the much higher Fe content residing in metal in NWA 10155. Lodranites may display pools of metal on a mm-scale, much coarser than the host silicates, an example being Elephant Moraine 84302 (Grady et al., 2014, p.213).

Figure 2. Three low-resolution snapshots taken with a stereomicroscope. The left and centre images are from the larger slice, the right-hand image is detail from the smaller slice. The slices are lightly polished, though not to a metallurgical finish, thus saw marks are visible in the metal. Abundant metal grains (Ni-Fe alloy, kamacite) are visible in all three, in a silicate -dominated groundmass. The left-hand image shows an area traversed by a secondary Fe oxyhydroxide (goethite, limonite) veinlet, presumed formed in the desert during prolonged weathering. All 3 images at 20X nominal magnification in unpolarized reflected light, long-axis field of view 6.4 mm.

Lodranites and acapulcoites

Research indicates that acapulcoites and lodranites are related, though the latter may have undergone greater metamorphism and partial melt removal. Acapulcoites tend to have chondritic mineralogies, as well as finer grain size, whereas lodranites are coarser and depleted in troilite, plagioclase or both (Floss, 2000). Examples are nicely illustrated by Grady et al. (2014, pp.200-214). These primitive achondrites yield very early dates. According to Keil and McCoy (2018) acapulcoites have been dated as old as 4563.1±0.8 Ma, and the lodranites as old as 4562.6±0.9 Ma. Both are seen as ancient residues of partial melting of a chondritic source, a process recorded in the distribution of trace elements in the metal phase (Hidaka et al., 2019). Relict chondrules have been noted in at least nine 9 acapulcoites. The acapulcoites are fine-grained (150-230 microns) with equigranular, achondritic textures, while lodranites are similar but coarser, (540-700 microns). The heat source for the acapulcoites and lodranites was internal, probably the short-lived radionuclide ²⁶Al, though some suggest it was shock melting. The acapulcoites experienced 1-4% partial melting, lodranites 5-10% or more (at 980-1170 and 1150-1200C, respectively). Cosmic ray exposure ages of 4-6 Ma and occurrence of both acapulcoite and lodranite in some meteorites indicate that the two groups share the same parent body, which was probably an S-type asteroid, no more than 100 km in diameter, and fragmented by an impact at 4556 Ma. These so-called primitive achondrites were formed from parent bodies that actually accreted later than those of differentiated achondrites, and so had less ²⁶Al, resulting in a lower level of differentiation, with partial melting arrested at circa 15 percent (Keil and McCoy, 2018).

More broadly, a review of available O isotope data from various methods (Greenwood et al., 2017) suggests that our meteorite collections come from about 110 asteroidal parent bodies (60 for iron meteorites, 35 for achondrites and stony irons and 15 for chondrites).

References

Floss,C (2000) Complexities on the acapulcoite-lodranite parent body: evidence from trace element distributions in silicate minerals. Meteoritics & Planetary Science 35, 1073-1085.

Grady,MM, Pratesi,G and Moggi-Cecchi,V (2014) Atlas of Meteorites. Cambridge University Press, 373pp.

Greenwood,RC, Burbine,TH, Miller,MF and Franchi,IA (2017) Melting and differentiation of early-formed asteroids: the perspective from high precision oxygen isotope studies. Chemie der Erde 77 no.1, 1-43.

Hidaka,Y, Shirai,N, Yamaguchi,A and Ebihara,M (2019) Siderophile element characteristics of acapulcoite-lodranites and winonaites: implications for the early differentiation processes of their parent bodies. Meteoritics & Planetary Science 54, 1153-1166.

Keil,K and McCoy,TJ (2018) Acapulcoite-lodranite meteorites: ultramafic asteroidal partial melt residues. Chemie der Erde 78 no.2, 153-203.

Graham Wilson, 19-20 March 2021

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