NWA 6136, a CO3 carbonaceous chondrite meteorite

NWA 6136, a CO3 carbonaceous chondrite

- meteorite mineralogy and textures

Figure 1. Two polished thin sections, with many small chondrules, seen in plain transmitted light. The two sections were prepared from the same thin slice. Each section displays an area of some 7.5 cm² for study, in a circa 3.8x2.2 cm compass. Parent slice from Blaine Reed.

"Rock of the Month #170, posted for August 2015" ---

The NWA 6136 meteorite

NWA 6136 was found in Morocco in 2008, in two pieces, total known weight 2.67 kg. It is classified as a CO3 (S2) (W3). The mineralogy is unequilibrated (Alan Rubin, in Meteoritical Bulletin 99) and crowded (70%) with small (0.15-0.2 mm) chondrules. The olivine shows variable composition (Fa_{1.5-33.3, mean 16.4}). Amoeboid olivine inclusions were also noted in the type thin section. The AOI, which occur in numerous CO meteorites, are typically 50-400 microns in size, with 45-70 volume percent olivine (Chizmadia et al., 2002). The Ornans group of chondrites are characterized by abundant small chondrules, the presence of refractory inclusions, and unequilibrated mineralogy. There are some 499 known, classified CO chondrites, of which 257 are type CO3 (Meteoritical Bulletin, 29 July 2015). At this time there are no MINLIB records on the NWA 6136 meteorite. Other notable NWA CO chondrites include NWA 2918, which contains a deep blue titanian spinel (Grossman et al., 2006) and the very fresh NWA 4530 (Bunch et al., 2010).

Figure 2. Two photomicrographs of chondrules. Left: A rather striking and atypical form of barred chondrule, apparently olivine set in abundant swirly glass, indicative of rapid cooling from a melt. Diameter 300 microns. Nominal magnification 200X, long-axis field of view 0.45 mm, plane-polarized transmitted light. Right: Another rather unusual chondrule, this one a variant of "porphyritic olivine chondrule". It is unusual in that one olivine crystal fills roughly 90% of the visible cross-section of the chondrule. The higher birefringence near the margins implies that the crystal is not homogeneous: the olivine composition varies from core to rim. Chondrule diameter circa 750 microns. Nominal magnification 100X, long-axis field of view 0.9 mm, cross-polarized transmitted light.

The CO chondrites contain abundant (13 volume percent) CAI (calcium aluminium inclusions, refractory inclusions), which are dominated by melilite, perovskite and spinel. The CAI may be somewhat altered, with melilite replaced by nepheline. Porphyritic olivine chondrules are abundant, with olivine of highly variable composition (Fa_1-60). Rubin (2011) observed that the CV, CK and CR chondrites have relatively larger chondrules, and common igneous rims on chondrules, and speculated that they may have formed in dustier parts of the solar nebula than other chondrites, including the CO chondrites with their small chondrules.

A study of ten CO chondrites suggests that they may be divided into a metamorphic sequence of types 3.0 to 3.7. In some CO3 stones, Ni-Fe metal has significant levels of Cr, Si and P, as in the least- metamorphosed ordinary chondrites and CM2 chondrites. There are traces of the carbides cohenite and haxonite, the first reported in carbonaceous chondrites. The carbides occur with pentlandite and magnetite, as in some LL3 chondrites (Scott and Jones, 1990).

The matrix is virtually anhydrous, composed of olivine and pyroxene, metal, magnetite and sulphides (Rubin, 1998; Grady et al., 2014, pp.23-25). This recent review of meteorite classes gives 8 examples of CO chondrites: Colony, Acfer 374, Ornans, Kainsaz, Felix, Lance, Allan Hills 77003 and Warrenton (Grady et al., 2014).

The CAI of CO chondrites have been studied in detail. Most CAI contain magnesium isotope ratios indicative of the incorporation of live radionuclide ²⁶Al into the CAI grains (Russell et al,, 1998), which indicates that these CAI formed at an early stage in the evolution of the solar nebula.

Figure 3. Two photomicrographs of chondrules. Left: a multi-domain barred olivine chondrule, 0.3 mm in diameter. Nominal magnification 200X, long-axis field of view 0.45 mm, cross-polarized transmitted light. Right: An atypically large and ornamented barred olivine chondrule, 0.75x0.65 mm in size. Note the curious partial outer rind, like material that escaped the cookie cutter... Nominal magnification 100X, long-axis field of view 0.9 mm, cross-polarized transmitted light.

A review of the two thin sections yields a rough visual mode of the sample as follows: well-defined porphyritic olivine chondrules (50%), dark fine-grained matrix (20%), troilite (10%), generally fine-grained CAI and AOI (amoeboid olivine inclusions, 9%), discrete coarser olivine grains in the matrix (4%), secondary oxide (goethite, 3%), barred olivine chondrules (2%), excentroradial pyroxene chondrules (1%), a dark inclusion of (?) a more primitive carbonaceous chondrite lithology with very few chondrules (1%), plus traces of porphyritic pyroxene-dominant chondrules, discrete, relatively coarse orthopyroxene and kamacite*. Since many of the features are rather fine-grained, it is no surprise that back-scattered electron imagery is in many ways more informative than the classic optical microscopy used in this description.

Figure 4. Two photomicrographs of a porphyritic olivine chondrule. Left: note the olivine, interstitial glassy mesostasis, and rounded opaque inclusions. Nominal magnification 100X, long-axis field of view 0.9 mm, plane-polarized transmitted light. Right: As above, seen in plane-polarized reflected light, revealing the presence of Ni-Fe metal (white), troilite (FeS, brownish) and secondary goethite (blue-grey).

Many of the chondrules display thin rims. A study of the ALH77307 CO3 stone indicated that some 80% of chondrules have at least partial fine-grained rims 10-110 microns thick, and that sheet silicates are most abundant in the chondrules and their rims (Itoh and Tomeoka, 2001). What little fresh metal remains is to be found in chondrules and in the scattered coarser silicate crystals, which may serve as "armour" to protect the metal from reaction, whether in the solar nebula, the parent body of the meteorite or, in the recent past, from weathering on the desert floor. In some cases, it is thought that troilite rims formed by reaction of the metal cores with sulphur-bearing gas (Imae and Kojima, 2000).

* Kamacite, meteoritic Ni-Fe metal alloy, which I believe has been recently and pedantically renamed "iron" by the nomenclature folks at the IMA, despite the historic, meteorite-specific context of this venerable name. I hope I am mistaken here or - if not - that meteoriticists ignore such deus ex machina, formulaic dogma.

Figure 5. Three photomicrographs of fine-grained inclusions. Note that, in each case, there is a thin rind of very fine-grained material around the irregular body. These are generally dark and perhaps largely sheet silicates darkened by secondary goethite. The clear rim on the CAI at right is perhaps a reaction rim of solar nebular origin. Left: lobate olivine-rich "amoeboid olivine inclusion" (AOI). Centre: a 600x350-micron CAI, largely melilite and fine-grained perovskite grains, next to a 350-micron excentroradial pyroxene chondrule. Right: A 450x300-micron CAI, (?) mostly fine-grained melilite, with a thin rim. In each case, nominal magnification 100X, long-axis field of view 0.9 mm, plane-polarized transmitted light (except: cross-polarized in centre image).

References

Bunch,TE, Irving,AJ, Rumble,D, Wittke,JH and Hupe,G (2010) Polymict CO3.05 chondrite Northwest Africa 4530: an anomalous oxidized sample from the regolith of the CO chondrite parent body. Meteoritics & Planetary Science 45, A27.

Chizmadia,LJ, Rubin,AE and Wasson,JT (2002) Mineralogy and petrology of amoeboid olivine inclusions in CO3 chondrites: relationship to parent-body aqueous alteration. Meteoritics & Planetary Science 37, 1781-1796.

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

Grossman,JN, MacPherson,GJ, Vicenzi,EP and Alexander,CMO (2006) A spectacular compound chondrule-CAI in Northwest Africa 2918, a new CO3.1 chondrite. Meteoritics & Planetary Science 41, A69.

Imae,N and Kojima,H (2000) Sulfide textures of a unique CO3-chondrite (Y-82094) and its petrogenesis. Antarctic Meteorite Research 13, National Institute of Polar Research, Tokyo, 349pp., 55-64.

Itoh,D and Tomeoka,K (2001) Phyllosilicate-bearing chondrules and clasts in the ALHA 77307 CO3 chondrite: evidence for parent-body processes. Antarctic Meteorites 26, NIPR, Tokyo, 183pp., 47-49.

Rubin,AE (1998) Correlated petrologic and geochemical characteristics of CO3 chondrites. Meteoritics & Planetary Science 33, 385-391.

Rubin,AE (2011) Why chondrites from different groups look different. Meteorite 17 no.3, 5-6.

Russell,SS, Huss,GR, Fahey,AJ, Greenwood,RC, Hutchison,R and Wasserburg,GJ (1998) An isotopic and petrologic study of calcium-aluminum-rich inclusions from CO3 meteorites. Geochimica et Cosmochimica Acta 62, 689-714.

Scott,ERD and Jones,RH (1990) Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites. Geochimica et Cosmochimica Acta 54, 2485-2502.

Graham Wilson, 29-31 July 2015, updated 03 August 2015 and 28-29 Jan. 2016

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