"Rock of the Month # 49, posted for July 2005" --- Sample GCW 1796, purchased from David New (Wilson, 1996). Photomicrograph of a barred olivine chondrule, in cross-polarized transmitted light, nominal magnification 100X, long-axis field of view 0.6 mm. Digital image, 29 June 2005.
The Moorabie meteorite is an unequilibrated ordinary chondrite with exceptionally diverse and well preserved chondrules, of which the most abundant by far are the porphyritic olivine chondrules. It is an attractive chondrite (e.g., Haag, 1997). The stone, a 1965 find (alias Moorabbie or Quinyambie) was recovered as a 14.04 kg mass found near Boolka, about 130 miles (209 km) north of Broken Hill. It is catalogued as an L3 stone (L3.6, S4-5: Grady, 2000, p.340). It is fairly well-studied, due to its somewhat unusual nature: a list of publications describing aspects of Moorabie is appended.
The small sample described herein is a covered thin section of a slice 18x12 mm (2.1 cm2) in size. It displays abundant Ni-Fe metal, circa 8 volume percent, disseminated and in thin discontinuous haloes around many well-preserved chondrules, in grains up to 0.7x0.4 mm in size. The thin section contains an estimated 92 volume percent chondrules of at least six types, a remarkably high abundance, in a sparse matrix dominated by metallic selvages. The chondrule types are listed in the table, together with a visual estimate of their relative proportions.
|Chondrule morphological type||Maximum dimensions||Abundance (vol.%)|
|Porphyritic olivine||1.6x1.4 mm||82%|
|Barred olivine||2.5x2.4 mm||3%|
|Excentroradial pyroxene||1.1x0.8 mm||3%|
|Granular orthopyroxene||0.9x0.8 mm||2%|
|Very fine-grained||3.0x2.8 mm||1%|
|Laminated pyroxene||0.9x0.7 mm||1%|
Moorabie is sufficiently unusual, with a high proportion of chondrules, that it has been identified with a small number of other ungrouped chondrites (Russell et al., 1998; Hutchison, 2004, pp.146-148,410-411). These low-FeO ordinary chondrites include four H-affinity chondrites and two of L-affinity, Moorabie and Suwahib (Buwah). Moorabie itself has chondrules averaging 0.65 mm in diameter, larger than normal for H chondrites, and somewhat more magnesian olivine (average Fo84.1). Oxygen isotopes are consistent with L or possibly LL chondrites, and it may overall be classified as an L3.8 (S4-5). There remains a possibility that this small set of chondrites represents a seldom-sampled parent body, distinct from the sources of the H, L and LL bodies (Hutchison, 2004, p.147).
The low-FeO chondrites are also discussed by Meibom and Clark (1999), who suggest that meteorite collections sample at least 135 different asteroids. As an unequilibrated ordinary chondrite, the stone has also been analysed for its content of the cosmogenic radionuclide aluminium-26. Calcium-aluminium inclusions have been documented in Moorabie, containing hibonite, spinel, ilmenite and sodalite, and there is evidence in the refractory phases of live 26Al incorporated into the meteorite at the time of its formation (Russell et al. 1996, 1997; Huss et al. 2001). Small inclusions of poorly-graphitized carbon may be associated with metal in such primitive chondrites (Mostefaoui et al. 2000). Moorabie also contains a troilite-rich clast with elongate chondrules indicative of shock melting (Fujita and Kitamura, 1992). Sulphides such as troilite and pentlandite are present, as is the plagioclase glass, maskelynite.
References on the Moorabie meteorite
FUJITA,T and KITAMURA,M (1992) Shock melting origin of a troilite-rich clast in the Moorabie chondrite (L3). Proc. NIPR Symposium on Antarctic Meteorites 5, 336pp., 258-269.
GRADY,MM (2000) Catalogue of Meteorites. Natural History Museum, London / Cambridge University Press, 5th edition, 690pp. plus CD-ROM.
HAAG,RA (1997) The Robert A. Haag Collection Field Guide of Meteorites. 12th Anniversary Edition, Tucson, AZ, 60pp.
HUSS,GR, MACPHERSON,GJ, WASSERBURG,GJ, RUSSELL,SS and SRINIVASAN,G (2001) Aluminum-26 in calcium-aluminum-rich inclusions and chondrules from unequilibrated ordinary chondrites. Meteoritics & Planetary Science 36, 975-999.
HUTCHISON,R (2004) Meteorites: a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press, 506pp.
MEIBOM,A and CLARK,BE (1999) Evidence for the insignificance of ordinary chondritic material in the asteroid belt. Meteoritics & Planetary Science 34, 7-24.
MOSTEFAOUI,S, PERRON,C, ZINNER,E and SAGON,G (2000) Metal-associated carbon in primitive chondrites: structure, isotopic composition, and origin. GCA 64, 1945-1964.
RUSSELL,SS, SRINIVASAN,G, HUSS,GR, WASSERBURG,GJ and MACPHERSON,GJ (1996) Evidence for widespread 26Al in the solar nebula and constraints for nebula time scales. Science 273, 757-762.
RUSSELL,SS, HUSS,GR, MACPHERSON,GJ and WASSERBURG,GJ (1997) Early and late chondrule formation: new constraints for solar nebula chronology from 26Al/27 Al in unequilibrated ordinary chondrites. Lunar and Planetary Science 28, 1209-1210.
RUSSELL,SS, McCOY,TJ, JAROSEWICH,E and ASH,RD (1998) The Burnwell, Kentucky, low iron oxide chondrite fall: description, classification and origin. Meteoritics & Planetary Science 33, 853-856.
WILSON,GC (1996) Mineralogical notes on miscellaneous samples: sulphidic charnockite, native gold, and the Moorabie and Millbillillie meteorites. TGSL Report 1996-101, 24pp.
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