Iron meteorite evidence for early formation and catastrophic disruption of protoplanets

Abstract

In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.1-1 Myr, and these collided more energetically to form planets. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5-200 km in diameter. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr(-1), increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed approximately 4,500 Myr ago in a 'hit-and-run' collision between Moon-to-Mars-sized protoplanets. This demonstrates that protoplanets of approximately 10(3) km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.

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

“…For this scenario, EET could have been derived from an independent, much smaller body, having a much faster cooling rate. A similar case for crystallization in genetically related, but distinct parent bodies has previously been made for Fuzzy Creek (Yang et al, 2007), and for Maria Elena (1935), which displays a Widmanstätten pattern when viewed by eye, but has been severely reheated with the formation of rounded taenite regions at kamacite/taenite boundaries. Because of possible minor differences in volatile components, such as S, in secondary metallic bodies, similar, but not necessarily identical crystallization paths would be expected for elements like HSE.…”

Diffusion‐driven kinetic isotope effect of Fe and Ni during formation of the Widmanstätten pattern

“…The reason is that thermal conductivity in a metallic core is much higher than in the overlying silicate mantle, so one would not expect large temperature gradients between samples from the central and peripheral regions of the core. However, several groups have shown that for IVA, the computed cooling rates vary from one meteorite to another and correlate with bulk Ni concentrations (Goldstein and Short 1967;Moren and Goldstein 1979;Rasmussen 1982;Rasmussen et al 1995;Yang et al 2007). Haack et al (1996a) suggested that after crystallization, the core was disrupted by impact and the resulting debris were reassembled by gravitation.…”

Coexistence pressure for a martensitic transformation from theory and experiment: Revisiting the bcc-hcp transition of iron under pressure

“…Thus, magneto-structural transformations and high-pressure states in iron attract enormous interest, especially in geophysics because iron is a primary constituent of the Earth's core, 29−163 many meteorites, [163][164][165][166][167][168][169][170][171][172] and, due to its properties and availability, most steels. 173 At low pressure P and temperature T , the α-phase of iron is a ferromagnet (FM) with the body-centered cubic (bcc) structure.…”