The terrestrial magma ocean: geochemical constraints

Abstract:

The study of meteorites shows that magma ocean stages are common in the early evolution of the solar system. Widespread melting events occurred very quickly after the beginning of solar system accretion in response to heating by short-lived radioactive elements. Later, they were produced by large energetic collisions of planetesimals. Lunar rocks collected during the Apollo missions provide a unique opportunity to investigate the process of magma ocean crystallization. Silicate reservoirs of different chemical compositions formed very quickly following the giant impact forming the Moon. For Mars, the long-term preservation of initial heterogeneities is clearly observed by the large variation in the abundance of the decay products of a number of short-lived, now extinct, radionuclides in relatively young rocks. The Earth’s magma ocean stage is much more difficult to investigate because early-formed chemical heterogeneities tend to be erased by efficient mantle mixing and rock samples from the first 500 Myr of Earth’s history (Hadean) are essentially missing.

Short- and long-lived radiogenic isotope systems are able to provide strong constraints on both the composition and the timing of the early-formed silicate reservoirs. Variability in 142Nd (formed through time by the radiogenic decay of 146Sm, half-life = 103 Myr) in Archean rocks points to an early Earth mantle that was quite possibly more chemically heterogeneous than the present-day mantle. Moreover, the differences between chondritic and terrestrial 142Nd/144Nd have been discussed in the past 15 years. Do they reflect an early silicate differentiation of the Earth or simply heterogeneity of the early solar system? Here we will present an overview of geochemical data measured in terrestrial samples that allow us to track and better constrain the magma ocean stage for the Earth. Decoupled Hf-Nd radiogenic isotope signatures measured in Archean samples from different locations reflect the crystallization of a deep magma ocean. The preservation of ancient heterogeneities through time will be discussed from 142Nd results measured in samples from different volcanic hotspots.