The Worden meteorite: A new ordinary chondrite fall from Michigan, USA

An ordinary chondrite fall in southeast Michigan, USA (near the crossroads hamlet of Worden in northeast Washtenaw County) penetrated the garage roof of a private home on 1997 September 1. The Worden chondrite comprises silicate matrix, mineral fragments, chondrules, chondrule fragments, and opaque primary phases. Electron microprobe analyses (olivine, Fa23.9; orthopyroxene, FS20.1, En78.8, W01) indicate diagnostic L-chondrite silicate mineral compositions. Recognizable chondrules and chondrule fragments constitute up to 42 vol%. Chondrule boundaries are readily discernable (especially where chondrules have rims of opaque material) but not sharp, and discrete plagioclase crystals are visible in the devitrified mesostasis of barred olivine chondrules; together, these characteristics suggest petrologic type S. The spatially averaged ensemble of shockrelated features (optical extinction of olivine grains, minor localized shock melt) suggests assignment of a shock stage of S3. The 26 Al and 22Na activities are typical for an L chondrite. Worden fell near the most recent solar minimum in 1997, and the 22Na production rate should have been at a level approaching the maximum levels due to solar modulation of the galactic cosmic-ray flux. The low value for the Worden 22Na activity relative to those observed in chondrite falls associated with the 1969 solar maximum and 1976 solar minimum suggests a relatively small preatmospheric size. The 60Co activity confirms the indication of a small body. The 56Co activity was essentially zero, indicating that none of the recovered meteorite contained material exposed to solar cosmic rays. The Worden chondrite is the fourth documented fall in Michigan, and the fourth stony meteorite recovered in Michigan; all other Michigan meteorites are finds, not observed falls, and are iron meteorites. All Michigan falls to date are ordinary chondrites. The three falls prior to Worden were Allegan (HS) fell 1899; Rose City (HS, brecciated, black (dark matrix)) fell 1921 ; and Coleman (L6, veined (shocked); Osborn et aI., 1997) fell 1994. Worden has a much lighter matrix than Rose City, and is less extensively thermally and shock metamorphosed than either Rose City (StOffler et aI., 1991) or Coleman.