Testing the chondrule-rich accretion model for planetary embryos using calcium isotopes

Citation:
Amsellam, E, Moynier F, Pringle EA, Bouvier A, Chen H, Day JMD.  2017.  Testing the chondrule-rich accretion model for planetary embryos using calcium isotopes. Earth and Planetary Science Letters. 469:75-83.

Date Published:

2017/05

Abstract:

Understanding the composition of raw materials that formed the Earth is a crucial step towards
understanding the formation of terrestrial planets and their bulk composition. Calcium is the fifth most
abundant element in terrestrial planets and, therefore, is a key element with which to trace planetary
composition. However, in order to use Ca isotopes as a tracer of Earth’s accretion history, it is first
necessary to understand the isotopic behavior of Ca during the earliest stages of planetary formation.
Chondrites are some of the oldest materials of the Solar System, and the study of their isotopic
composition enables understanding of how and in what conditions the Solar System formed. Here we
present Ca isotope data for a suite of bulk chondrites as well as Allende (CV) chondrules. We show that
most groups of carbonaceous chondrites (CV, CI, CR and CM) are significantly enriched in the lighter Ca
isotopes (δ44/40Ca= +0.1 to +0.93) compared with bulk silicate Earth (δ44/40Ca= +1.05 ± 0.04,
Huang et al., 2010) or Mars, while enstatite chondrites are indistinguishable from Earth in Ca isotope
composition (δ44/40Ca = +0.91 to +1.06). Chondrules from Allende are enriched in the heavier
isotopes of Ca compared to the bulk and the matrix of the meteorite (δ44/40Ca = +1.00 to +1.21).
This implies that Earth and Mars have Ca isotope compositions that are distinct from most carbonaceous
chondrites but that may be like chondrules. This Ca isotopic similarity between Earth, Mars, and
chondrules is permissive of recent dynamical models of planetary formation that propose a chondrulerich
accretion model for planetary embryos.

DOI:

10.1016/j.epsl.2017.04.022