Mount Magnet

Iron, IAB complex, sHH subgroup

Found 1916
28° 2′ S., 117° 58′ E. Two fragments constituting a single sickle-shaped mass having a combined weight of 16.5 kg were found in Western Australia, about 10 km east of Mount Magnet (Buchwald, 1975). The mass exhibits significant terrestrial weathering with no evidence of fusion crust or heat-affected zone. Mount Magnet is classified structurally as a plessitic octahedrite (Opl) with kamacite needles of a few µm in size. This iron was initially classified as geochemically anomalous, but has since been included in the high-Au, high-Ni subgroup (sHH) of the IAB complex (Wasson and Kallemeyn, 2002).

In a study involving Mo and W isotope compositions for the IAB complex, Worsham et al. (2017) determined that the Mo isotopic composition of the sHL and sHH subgroups are identical, but differ from that of the sLL, sLM, and sLH subgroups. Moreover, they calculated the formation ages of these irons based on W isotopes, and determined that the sHL and sHH subgroups formed significantly earlier than the MG, sLL, and sLM subgroups. From this study they concluded that the MG and the sLL, sLM, and sLH subgroups formed in distinct impact-melt pools on at least two distinct asteroids, whereas the sHL and sHH subgroups formed by fractional crystallization associated with core formation resulting from internal radiogenic heating on one or more separate parent bodies (see schematic diagram below).
Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 164 (2017)
‘Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex’
(https://doi.org/10.1016/j.epsl.2017.02.044) In a study of the IAB subgroups, employing precise Mo, W, and Os isotope data along with HSE and other literature data, Worsham et al. (2017) ascertained that the three sHH irons in the study (ALHA80104, Kofa, and Mount Magnet) have indistinguishable µ97Mo isotopic compositions with an average value of 24 (±3).
click on photo for a magnified view

Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 160 (2017)
‘Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex’
(https://doi.org/10.1016/j.epsl.2017.02.044) In another analysis of siderophile elements in IAB irons, Worsham et al. (2016) revealed that Mount Magnet has a more fractionated HSE pattern compared to the other two sHH samples in their study, Kofa and ALHA80104, both of which have nearly identical HSE patterns and which are somewhat similar to two late-crystallized IIIB irons (see diagram below, where Grant is a dashed line and Chupaderos is a dotted line). Based on this analysis, they suggest that Mount Magnet may have formed from a distinct parental melt on a common parent body.
Diagram credit: Worsham et al., GCA, vol. 188, p. 268 (2016)
‘Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group’
(https://doi.org/10.1016/j.gca.2016.05.019) The interior structure of Mount Magnet is ataxitic, containing numerous ribbons of schreibersite, small grains and clusters of silicates, and monocrystalline grains of troilite—each of these inclusions are often rimmed by kamacite. Both small pockets and skeletal crystals of schreibersite are also present, the former usually surrounded by wide kamacite rims. To learn more about the relationships within the IAB complex and among other iron chemical groups, see the Appendix, Part III. The photo shown above is an 11.32 g partial slice of Mount Magnet.