Shock‐thermal history of the Agoudal (IIAB) iron meteorite from microstructural studies

Abstract

The Agoudal IIAB iron meteorite exhibits only kamacite grains (~6 mm across) without any taenite. The kamacite is homogeneously enriched with numerous rhabdite inclusions of different size, shape, and composition. In some kamacite domains, this appears frosty due to micron‐scale rhabdite inclusions (~5 to 100 μm) of moderate to high Ni content (~26 to 40 wt%). In addition, all the kamacite grains in matrix are marked with a prominent linear crack formed during an atmospheric break‐up event and subsequently oxidized. This feature, also defined by trails of lowest Ni‐bearing (mean Ni: 23 wt%) mm‐scale rhabdite plates (fractured and oxidized) could be a trace of a pre‐existing γ–α interface. Agoudal experienced a very slow rate of primary cooling ~4 °C Ma−1 estimated from the binary plots of true rhabdite width against corresponding Ni wt% and the computed cooling rate curves after Randich and Goldstein (1978). Chemically, Agoudal iron (Ga: 54 ppm; Ge: 140 ppm; Ir: 0.03 ppm) resembles the Ainsworth iron, the coarsest octahedrite of the IIAB group. Agoudal contains multiple sets of Neumann bands that are formed in space and time at different scales and densities due to multiple impacts with shock magnitude up to 130 kb. Signatures of recrystallization due to postshock low temperature mild reheating at about 400 °C are also locally present.

Metallographic Cooling Rate and Petrogenesis of the Recently Found Huoyanshan Iron Meteorite Shower

“…Magmatic iron meteorites show significant fractionation as indicated by correlations of Ni with other highly siderophile elements, such as Ir and Au, formed during crystallization of metallic cores in asteroids (Goldstein et al., 2009; Scott, 1972; Wasson & Kimbeblin, 1967). The core origin is generally consistent with the slow cooling rates of many groups owing to being covered by a silicate layer up to tens of kilometers in thickness, such as 3–10°C/Myr for IID and 4–7°C/Myr for IIAB (Ghosh et al., 2016; Ray et al., 2019; Yang et al., 1997). A few magmatic groups, that is, IIIAB (56–338°C/Myr), IVA (100–6600°C/Myr) and IVB (475–5000°C/Myr), have fast and broad ranges of cooling rates (Goldstein et al., 2014; Yang & Goldstein, 2006; Yang et al., 2007, 2008), most likely due to stripping of the silicate mantle by early catastrophic impact (Yang et al., 2007).…”