Innisfree meteorite, Minburn County, Alberta, Canadai

Regional Level Types
Innisfree meteorite	Meteorite Fall Location
Minburn County	County
Alberta	Province
Canada	Country

Ordinary chondrite,brecciated (L5; S3; W0)
Fell, 6 Feb 1977; 4.58 kg

At 02:17:38 UT a bright fireball was photographed by cameras of the Canadian Meteorite and Recovery Project. Analysis indicated that a likely meteorite would have fallen ~140 km E of Edmonton. On February 17, only a few hours after a search had begun, a 2 kg chondrite was found lying on the snow where it had bounced from the hardened soil. In April, after the snow had melted 8 additional smaller pieces were found in the general area by both researchers and a local farmer. Photographs indicated that the meteorite had entered the atmosphere at 14.2 km/s and had been photographed at heights of 20-62 km with fragmentation occurring at the lower altitudes. A pre-impact orbit was determined (a= 1.8 a.u., e= 0.44, P= 2.4 yr) which indicated that the meteoroid was a orbital outlier of the asteroid belt. While scientific networks and amateur cameras have made determinations of pre-impact meteoroid orbits a much more common occurrence in today's world, Innisfree was only the third meteorite with an orbit referenced to a determinative photographic record. These 3 meteorites (Pribram [1959], Lost City [1961],Innisfree [1975]) made it evident what had already been suspected — most meteorites are indeed orbital stragglers from the asteroid belt.

The meteorite itself is somewhat unusual as it appears to be a chondritic breccia with clasts of varying metamorphic grade and, sometimes, darker colors within the overall light grey host. Chondrules, chondrule fragments and lithic fragments (up to 2 mm in diameter) account for ~50 vol% of the meteorite. Olivine and pyroxene are the predominant silicates with minor, well-crystallized plagioclase present in the matrix. Both orthopyroxene and inverted clinopyroxene are found as well as devitrified glass. Troilite and lesser amounts of Fe-Ni metal account for less than 1 vol% of the meteorite. Accessory chromite, ilmenite, and phosphates are found only as minor accessories. Some devitrified glass is found within chondrules and occasional grains of troilite contain immiscible metal droplets. Nickel abundances within Fe-Ni metal vary wildly (2—57 wt%Ni) with a major peak at 6-7 % and a minor peak at 52-53%.

The L (relatively low in total iron) chondrites are the largest group of ordinary chondrites and represent ~40% of fully classified falls. The main mass of the Innisfree meteorite has been retained by the Geological Survey of Canada.

Nota Bene: The current 'L5' classification listed at the Meteoritical Bulletin is based upon the work by Rubin (1990). Rubin's classification relied heavily upon a 24 point count analyses of olivine (n=9) and kamacite (n=15) with both minerals exhibiting very little compositional scatter in the studied samples. However, Smith (1980) classified Innisfree as an 'LL5' (very low in total iron) chondrite as he had derived a slightly higher fayalite value for olivine (Fa 27.1 mol%) than Rubin had derived later (Fa 25.3 mol%). However, a very wide range for Ni in metal also reported by Smith was based upon over 500 spot analyses. This variability — as well as the Ni peak at ~52 wt%Ni (suggestive of possible tetrataenite) — may be indicators of heterogeneity in the actual meteorite that is not captured under its current simple 'L5' label.

References

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