Silica and pyroxene in IVA irons; possible formation of the IVA magma by impact melting and reduction of L-LL-chondrite materials followed by crystallization and cooling

Group IVA irons: New constraints on the crystallization and cooling history of an asteroidal core with a complex history

“…These findings support a model IVA core with an initial S content of $3 wt.% (Model B), instead of 0.5 wt.% (Model A) as proposed by Wasson and Richardson (2001) and Wasson et al (2006), which was based on fitting a quadratic (Table 3). …”

“…About half the members of the group have seen moderate to heavy shock, and a few members of the group contain silicates. It has been argued that the silicates were introduced via a high temperature impact (Wasson et al, 2006).…”

“…Depletion in volatile siderophile elements may have been caused by preaccretionary processes, such as nebular evaporation/ condensation, acting on precursor materials (e.g., Sears, 1978). Alternately, the depletions in the IVA irons may have resulted from processes that occurred in the parent body prior to core formation, such as oxidation/reduction reactions, or impact volatilization (Wasson and Richardson, 2001; Ruzicka and Hutson, 2006;Wasson et al, 2006).…”

“…Sulfur, and to a lesser extent P, play important roles in controlling the partitioning of siderophile elements between solid and liquid metal. Some studies have estimated that the initial S content of the IVA system was $0.5 to 3 wt.%, based on inter-element variations of elements such as Ir-Au (Willis and Goldstein, 1982;Jones and Drake, 1983;Wasson and Richardson, 2001;Chabot, 2004;Wasson et al, 2006). Ga and Ge variations, however, cannot be easily reconciled with such low initial S. The behavior of these two volatile, moderately siderophile elements is more consistent with considerably higher initial S (e.g., $9 wt.%; Chabot, 2004).…”

“…The proposed scrambling and reassembly of partially differentiated metal into a new core could potentially account for the differential cooling rates, yet does not well account for the good correlation between cooling rate and extent of chemical fractionation. Wasson et al (2006) speculated that the methodology to constrain metallographic cooling rates may be flawed, and that IVA cooling rates were actually generally constant. Yang et al (2008) concluded that the IVA irons cooled below 650°C at highly variable rates of 100-6600 K/Myr, where cooling rate decreased with increasing Ni content.…”