Meteorite evidence for partial differentiation and protracted accretion of planetesimals

Abstract

Modern meteorite classification schemes assume that no single planetary body could be source of both unmelted (chondritic) and melted (achondritic) meteorites. This dichotomy is a natural outcome of formation models assuming that planetesimal accretion occurred nearly instantaneously. However, it has recently been proposed that the accretion of many planetesimals lasted over ≳1 million years (Ma). This could have resulted in partially differentiated internal structures, with individual bodies containing iron cores, achondritic silicate mantles, and chondritic crusts. This proposal can be tested by searching for a meteorite group containing evidence for these three layers. We combine synchrotron paleomagnetic analyses with thermal, impact, and collisional evolution models to show that the parent body of the enigmatic IIE iron meteorites was such a partially differentiated planetesimal. This implies that some chondrites and achondrites simultaneously coexisted on the same planetesimal, indicating that accretion was protracted and that apparently undifferentiated asteroids may contain melted interiors.

The thermal evolution of planetesimals during accretion and differentiation: consequences for dynamo generation by thermally-driven convection.

“…Bryson, Neufeld, and Nimmo (2009) used these observations to constrain the CV parent body size R 0 > 220 km growing to R p > 270 km later. Meteorites from the H chondrites (Bryson, Weiss, et al, 2019), mantle-hosted IIE irons (Maurel et al, 2020) and L/LL chondrites (Shah et al, 2017) also experienced a planetary dynamo field but these fields were younger and post-dated any thermally driven dynamo fields. Therefore these meteorites likely experienced magnetic fields driven by core crystallization on their parent bodies.…”

“…The youngest period of magnetism from ∼65-250 Myr after solar system formation has been linked to dynamo fields generated during core crystallization on the parent bodies (Bryson et al, 2015;Bryson, Weiss, et al, 2019;Maurel et al, 2020). The exact mode of core crystallization in planetesimals is uncertain and may proceed from a nucleus outward (as with the Earth's inner core) or inwards from the core-mantle boundary (CMB), depending predominantly on the size of core and its light element content (Williams, 2009).…”

A Long‐Lived Planetesimal Dynamo Powered by Core Crystallization

“…It also implies that the field must have been directionally stable over the period of magnetization acquisition. A crustal field, resulting from the magnetization of an H‐chondrite‐like crust acquired during earlier magnetic activity of the parent body, would also have been orders of magnitude too weak to explain the results obtained (Maurel et al., 2020). With an 40 Ar/ 39 Ar age of 159 ± 9 Ma after CAI‐formation (Bogard et al., 2000), As such, Miles has the potential to extend the known paleomagnetic record for the IIE body by > 60 Ma.…”

“…The timing of the pallasite records is based on conductive cooling simulations assuming a fully differentiated 200‐km radius parent body and so is dependent on model parameters as indicated by the arrows. Data from (Biggin et al., 2015; Bryson et al., 2015, 2019; Carporzen et al., 2011; Garrick‐Bethell et al., 2017; Gattacceca et al., 2016; Johnson et al., 2015; Maurel et al., 2020; Nichols et al., 2016; Nichols, 2017; Shah et al., 2017; Tarduno et al., 2010; Wang et al., 2017; Weiss et al., 2002; 2008).…”

“…Buoyancy‐driven segregation of the molten metal‐silicate mixture would have been prevented by exposure to near‐surface temperatures and rapid cooling (>2.5°C h −1 at 850–1000°C as indicated by the presence of silicate glass; Ruzicka, 2014). Following the impact, the source regions for the meteorites would have been buried under tens of km of material (Maurel et al., 2020) to explain the presence of metallographic microstructures that form at slow cooling rates (<10,000°C Ma −1 below ∼600°C, Ruzicka, 2014).…”