Geologic processes on asteroids in the early Solar System: A perspective from ordinary chondrites

Dr Rhian Jones, School of Earth, Atmospheric and Environmental Sciences, University of Manchester

Ordinary chondrites are the most common type of meteorite to land on the Earth: there are tens of thousands of ordinary chondrites in the world’s meteorite collections. There are around 900 observed falls of ordinary chondrites, including the famous Chelyabinsk meteorite that fell in Russia in February 2013. Asteroid Itokawa, the only asteroid from which samples have been returned to Earth, also consists of ordinary chondrite material. Ordinary chondrites record the geologic history of the first few tens of million years of Solar System history. Their constituent chondrules and other components represent some of the first solid particles that appeared in the Solar System. These pieces of dust aggregated to form asteroids, and asteroids were subsequently heated as a result of the decay of short-lived radioisotopes and impacts. Here I will focus on recent studies of metamorphism and metasomatism that took place on the parent body (or bodies) of the LL group of ordinary chondrites. Previous work has shown that peak temperatures of metamorphism were around 950 °C, and cooling rates were of the order of degrees per million years. Also, most previous work has assumed that throughout much of the heating period there were no fluids present. Our studies of primary and secondary feldspar, and the secondary phosphate minerals merrillite and apatite, have shown that effects of metasomatism are pervasive throughout the metamorphic sequence. Fluids were dry and chlorine-rich. We suggest that fluids are either derived from the interior of the asteroid where partial melting may occur, or they may be derived from melts formed during impacts. These observations lead us to re-evaluate models for the thermal histories of ordinary chondrite parent bodies, and help to interpret the behaviour of volatile elements in the inner Solar System in the planet-forming epoch.

▲ Up to the top