standby for seemore downs 001 photo
Found April 1991
30° 35' S., 125° 13' E.

When found in Western Australia, this meteorite was composed of several large fragments and four smaller pieces having a combined weight of 429 g. Seemore Downs might be a chemically extreme member of the L or LL ordinary chondrite groups, or it could be derived from its own unique parent body with properties intermediate to, or overlapping with, both the L and the LL parent bodies.

Previous parameters used to delimit an L/LL group from the L and LL groups include the siderophile element concentrations, the fayalite content in olivine, and the cobalt concentration in kamacite. New studies utilizing cluster anaysis statistics on the minor and trace element data conclude that the L/LL meteorites are chemically more closely related to each other than they are to either the L or LL group. Furthermore, the enriched REE concentration and the negative europium anomaly, which are found only in the L/LL meteorites, easily distinguishes them from the other groups (Friedrich and Lipschutz, 2001). Moreover, it was ascertained by Heck et al. (2009) that the O-isotopic compositions and the major and minor elemental compositions overlap between the L and LL chondrite groups. Since these meteorites do not fit into either of these two groups, they provide evidence for the existence of a separate L/LL, low total-Fe, chondritic parent body. The gas-retention age distribution determined for a set of 12 L/LL chondrites by Wasson and Wang (1991) revealed a different age range for each L, LL, and L/LL group. This suggests that meteorites from these chondrite groups originated from at least three separate parent bodies. At the close of 2009, the Meteoritical Bulletin Database has listed 102 meteorites classified as members of the L/LL group, although a portion of these could reflect an inability of the classifier to adequately distinguish between the L and LL groups.

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio, grain density, and magnetic susceptibility, and these Amean values all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group.

standby for amean diagram
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3)

Other members of this intermediate chondrite group include L/LL3 Inman, L/LL4 Bjurböle, L/LL5 Knyahinya, and L/LL6 Naryilco. The specimen of Seemore Downs 001 shown above is an 8.9 g partially crusted fragment.