Paleomagnetic Evidence for a Partially Differentiated Ordinary Chondrite Parent Asteroid

Abstract

The textures and accretion ages of chondrites have been used to argue that their parent asteroids never differentiated. Without a core, undifferentiated planetesimals could not have generated magnetic fields through dynamo activity, so chondrites are not expected to have experienced such fields. However, the magnetic remanence carried by the CV chondrites is consistent with dynamo‐generated fields, hinting that partially differentiated asteroids consisting of an unmelted crust atop a differentiated interior may exist. Here, we test this hypothesis by applying synchrotron X‐ray microscopy to metallic veins in the slowly cooled H6 chondrite Portales Valley. The magnetic remanence carried by nanostructures in these veins indicates that this meteorite recorded a magnetic field over a period of tens to hundreds of years at ∼100 Myr after solar system formation. These properties are inconsistent with external field sources such as the nebula, solar wind, or impacts, but are consistent with dynamo‐generated fields, indicating that the H chondrite parent body contained an advecting metallic core and was therefore partially differentiated. We calculate the thermal evolution of the chondritic portions of partially differentiated asteroids that form through incremental accretion across 105 to 106 years, finding this can agree with the measured ages and cooling rates of multiple H chondrites. We also predict that the cores of these bodies could have been partially liquid and feasibly generating a dynamo at 100 Myr after solar system formation. These observations contribute to a growing body of evidence supporting a spectrum of internal differentiation within some asteroids with primitive surfaces.

Constraints on the Distances and Timescales of Solid Migration in the Early Solar System from Meteorite Magnetism

“…This differs from the CV, H (high-metal ordinary chondrites) and R (Rumuruti-like) chondrites, the parent bodies of which have been suggested to have been partially differentiated (Carporzen et al 2011;Bryson et al 2019b;Cournede et al 2020) and all have members that were heated to high temperatures on their parent bodies for extended time periods (almost certainly 500˚C, Bonal et al 2016, and possibly ∼900˚C if the CV and CK chondrites originate from the same parent body, Greenwood et al 2010). As such, we disfavor core dynamo activity as the origin of the remanence we measured in WIS 91600.…”

Meteorite evidence for partial differentiation and protracted accretion of planetesimals

“…Although individual CZ islands (~10 to 150 nm in size) are not resolved, we can estimate the bulk NRM of micrometer-sized regions of the CZ by comparing the XMCD data to that expected for an underlying Maxwell-Boltzmann distribution of the islands' magnetization directions ( Fig. 3; see Materials and Methods and text S3) (14).…”

“…Our experimental protocol and approach to data processing builds upon that described in (14). The amount of absorbed circularly polarized x-rays depends on the angle between the helicity of the x-ray and the local magnetization direction, as well as contributions from the sample work function, elemental concentration of iron, and oxidation state of the element of interest.…”

Influence of redox processes on the germanium isotopic composition of ordinary chondrites

“…This supports the hypothesis that OCs can represent aggregates of early formed achondritic, chondritic, and chondrule materials Lichtenberg et al (2018). argued that OC parent bodies can form from early impact on a preheated (i.e.,26 Al decay) and partially differentiated planetesimal(Bryson et al 2019). Thus, the negative δ 74/70 Ge values of the bulk sample and all separated OC phases, compared with positive values for magmatic iron meteorites and Earth silicate reservoirs, imply a light isotope-enriched environment for OC formation, i.e., an impact plume environment.…”