Investigating The Impact-induced Magnetic Fabric In The Allende Chondritic Meteorite

Meteorites can record ancient magnetic fields when they form. These recordings of ancient magnetic fields can be retained for billions of years and can survive a meteorite’s entry into and through the Earth’s atmosphere. Unravelling the magnetic recordings of primitive meteorites that formed within the first few million years of our Solar System, can help us to understand the role of magnetic fields in the formation of the Solar Nebular.

There are thought to be several sources of the potential magnetic field, e.g., Solar Nebular fields, including powerful X-wind fields, impact-generated plasma-fields, and short-lived planetesimal (small planets > 80 km in diameter) dynamo fields drive by the decay of Al²⁶ that are physically similar to the Earth’s geodynamo. All of the potential sources are based on theories that are hotly debated: Solar Nebular fields are argued to be too weak, whilst the very existence of impact-generated plasma-fields is doubted.

Planetesimal dynamo fields are reliant on there having been partially or fully differentiated planetesimals, e.g, denser elements in the middle of the planetesimal with lighter elements on the surface, like the Earth, however, this theory disagrees with petrographic evidence from very early so-called primitive chondrites, which does not support differentiation. There is also the question of the magnetic recording mechanism; the magnetic fields in the early Solar Nebular are very weak (roughly the same as the current Earth’s field on the surface); some other physical mechanism is required to record these weak magnetic fields, e.g., heat, shock etc.

In our study (Muxworthy et al., 2017), we propose a new method of inducing magnetic fields in primitive chondrites based on microscopic differential heating during impacts. Recent models of an impact-induced mechanism for the lithification of meteorites propose the matrix of ‘unshocked’ chondrites may have been rapidly heated and cooled, reaching >1000 K (Bland et al., 2014). This mechanism for heating acts on the microscopic scale, and is brief (<10 s), resulting in no macroscopic shock textures, however, the material is rapidly heating making it possible to thermally record weak, ambient magnetic fields, i.e., the magnetization recorded is not a shock-induced process or a recording of a plasma field, but a simple thermomagnetic recording like those recorded by lavas.

Our theory is supported by new measurements made on the Allende primitive chondritic meteorite. As impacts generate a preferred orientation of the crystals inside a meteorite, a so-called crystallographic fabric, we searched for a similar magnetic fabric and found a very strong one. The most likely source for such a strong magnetic fabric is an impact. We also determined the intensity of the ancient magnetic field to be on the order of ~6 µT, of the same order of the early Solar Nebular field as predicted by models.

Our proposed new model for the magnetic recordings found in primitive meteorites, both agrees with petrographic evidence, i.e., it does not require planetesimal dynamos, and provides a physical recording mechanism.

The study, Evidence for an impact-induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization was recently published in the journal Meteoritics & Planetary Science.