7:37 PM 2024-11-30
Aba Panu L3 chondrite meteorite
The Aba Panu meteorite fall
an L3.6 chondrite, a meteorite shower in Nigeria in 2018
Figs. 1-2:
Two faces of a thin slice of the Aba Panu meteorite. Slice is 38x19x2 mm, 6.0 grams. This slice displays abundant pale, rounded cross sections of chondrules, 1-2 or occasionally 3 mm in diameter. The matrix is dull, medium-grey, and flecked with abundant flakes of silvery metal (kamacite). It may be my eyes, but what is less evident is the similar abundance of tawny sulphide (troilite) which is obvious under the microscope. Due to the metal content, a pen magnet is strongly attracted to the slice. Sample from Blaine Reed, 2023. A smaller, 4.0 gram slice was used to make a polished thin section for closer study (Figs. 3-9).
"Rock of the Month # 282, posted for December 2024" ---
Meteorite showers
occur when an incoming meteoroid
fragments in the Earth's atmosphere and
a large number of individual pieces (often only the size of peas)
survive to reach the ground.
A large fireball detonated over Nigeria on the afternoon of 19 April 2018, and a meteorite shower of many stones fell,
the largest apporoaching 40 kg in size. The stones are hard and largely lack fusion crust.
Many chondrules are visible on sawn surfaces. The stone contains clasts of L6, L5 and achondritic material.
However, the bulk is an unequilibrated chondrite (both olivine and orthopyroxene
compositions vary widely) and there is abundant metal and sulphide, plus trace chromite.
L chondrites are the second most-commonly recovered kind of meteorite in our collections, worldwide.
According to the Meteoritical Bulletin,
as of 22 October 2024, there were 76,553 verified meteorites,
of which 27,222 (35.6%) were L chondrites.
Only the related H chondrites are slightly more abundant in our collections
(29,017 meteorites, 37.9%).
Because Aba Panu is a relatively recent fall, only a few descriptions have been published to date.
However, being abundant (the TKW or total known weight is reported as at least
160 kg). and readily available at a reasonable price for a fresh fall, much more work will doubtless
be done on this meteorite.
The original description, submitted by L.A.J. Garvie, affirming the classification, appeared in the Met.Bull. (Gattacecca et al., 2020).
Physical properties have been tested, relevant to understanding the behaviour of
the parent asteroid(s) of the L chondrites (Rabbi et al., 2021).
It was included in a survey of the bulk mineralogy, water content and hydrogen isotope chemistry
of unequilibrated chondrites, alongside famous, much-studied
meteorites such as Semarkona and Bishunpur, Chainpur and Mezo-Madaras (Grant et al., 2023).
Figures 3-9 illustrate some textures in the thin section, at 50X magnification, long-axis field of view 1.7 mm, 3-5 in crossed-polarized, transmitted light, 6-8 in plane-polarized, reflected light.
Figs. 3-5: Left: large barred olivine chondrule.
Middle: porphyritic olivine chondrule.
Right: an unusual object (a "layered chondrule" or, what? - the only example noted in the 4.5 cm2 thin section) in which there is a distinct core and rim.
See also Fig. 9.
This is distinct from so-called compound chondrules (well-described in the literature, e.g., Ciesla et al., 2004; Hutchison, 2004, pp.57-59) which may represent
pairs of chondrules which collided and welded together prior to the aggregation of the parent body, an estimated 2.4 to 5% or more of all chondrules. Aba Panu also displays what dealers and collectors call armoured chondrules, a common occurrence where a thin film of metal and sulphide coats the silicate-dominated body of a chondrule (Hutchison, 2004, pp.132-133; Lauretta and Killgore, 2005, p.67; and shown here in Fig. 8). All these little (mm-scale) objects are evidence of processes in the early solar nebula and/or the coalescing, evolving and colliding
(asteroidal) parent bodies.
Figs. 6-8: all in reflected light, to reveal the nature of minerals that are opaque, even in a slice just 0.03 mm thick.
Left: metal (kamacite) partly surrounded by sulphide (troilite, FeS). These minerals are scattered through the fine-grained matrix of the rock, and often form thin partial rims on chondrules.
Middle:
one angular metal grain and smaller troilites. Note also the cloud of tiny spherules (more troilite than metal)
in the host silicate, an indication of intensive shock and partial melting of the material,
most probably during an impact event on the parent body.
Right: example of an armoured chondrule, with a thin, partial covering of troilite and metal.
Parent bodies of meteorites
A massive influx of L chondrite material happened at 466 Ma, in the middle Ordovician. The rusty relicts of the fragments that survived a fall to Earth are known as "fossil meteorites", and have most notably been found in limestone quarries in southern Scandinavia.
This short, sharp spike in the rate of meteorite infall
most likely reflects the breakup of a large asteroid in the main belt and, even today, material from the breakup is >20% of all falls. The Massalia collisional family of asteroids is the only plausible source of the L chondrites (Marsset et al., 2024).
Currently, and for some decades now, some 6% of meteorites are recognized as originating on the Moon, Mars or asteroid 4 Vesta.
- however, some 70% of meteorites come from 3 recent breakups of large (>30 km diameter) asteroids that occurred in late Miocene (5.8 Ma, 7.6 Ma: H chondrites) and late Eocene (roughly, <40 Ma: L chondrites) time. The breakups involved the Karin family, and the Koronis and Massalia families, and these events account for the predominance of H chondrites and L chondrites in our meteorite collections. The young families are unique amongst asteroids for a very high abundance of small fragments. The 40 Ma L chondrite event added to the paleometeorites of the L clan whose traces are preserved in those Ordovician limestones (Broz et al., 2024).
Fig. 9:
a second view of the strange object in Figure 5.
The body is circa 1.9x1.2 mm in plane of section, the core being 1.3x0.7 mm, surrounded by a rim some 0.25 mm thick.
The core is featureless, but for a radial fracturing pattern, and may well be glass.
The outer rim is finely crystalline. At higher magnification it appears to be composed
of short prismatic crystals, fairly high relief, apparently colourless and of moderate birefringence, the prisms of inclined extinction, and length-slow orientation. Diopside (?).
50X magnification, long-axis field of view 1.7 mm, in crossed-polarized, transmitted light,
The polarizers are very slightly uncrossed, otherwise the evidently isotropic core of the object would appear black here.
The core is relatively pure: in reflected light it appears almost as a void, since it lacks the disseminated metal and sulphide common elsewhere in the meteorite.
REFERENCES
Broz,M, Vernazza,P, Marsset,M, DeMeo,FE, Binzel,RP, Vokroulicky,D and Nevsomy,D (2024) Young asteroid families as the primary source of meteorites. Nature 634, 566-571 (16 October).
Ciesla,FJ, Lauretta,DS and Hood,LL (2004) The frequency of compound chondrules and implications for chondrule
formation. Meteoritics & Planetary Science 39, 531-544.
Gattacceca,J, McCubbin,FM, Bouvier,A and Grossman,J (2020) The Meteoritical Bulletin, No.107.
Meteoritics & Planetary Science 55, 460-462.
Grant,H, Tartèse,R, Jones,R, Piani,L, Marrocchi,L, King,A and Rigaudier,T (2023) Bulk mineralogy, water abundance, and hydrogen isotope composition of unequilibrated ordinary meteorites. Meteoritics & Planetary Science 58, 1365-1381.
Hutchison,R (2004) Meteorites: a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press, 506pp.
Lauretta,DS and Killgore,M (2004) A Color Atlas of Meteorites in Thin Section. Golden Retriever Publications / Southwest Meteorite Press, 301pp.
Marsset,M, Vernazza,P, Broz,M, Thomas,CA, DeMeo,FE, Burt,B, Binzel,RP, Reddy,V, McGraw,A, Avdellidou,C, Carry,B, Slivan,S and Polishook,D (2024) The Massalia asteroid family as the origin of ordinary L chondrites. Nature 634, 561-565 (16 October).
Rabbi,MF, Garvie,LAJ, Cotto-Figueroa,D, Asphaug,E, Khafagy,KH, Datta,S and Chattopadhyay,A (2021)
Understanding asteroidal failure through quasi-static compression testing and 3-D digital image correlation of the Abu Panu (L3) chondrite.
Meteoritics & Planetary Science 56, 2131-2143.
![slice side 1 [393 kb]](http://www.turnstone.ca/AP-HS1.jpg)
![slice side 2 [348 kb]](http://www.turnstone.ca/AP-HS2.jpg)
