Does the Earth have a chondritic rare-earth element composition? A study using combined cerium - neodymium isotope data to understand the formation of planets in the inner solar system

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The assumption that the chemical composition of the Earth is similar to that of the bulk solar system is key to our understanding of how the Earth formed [1- 3]. Recently detected super-solar system neodymium-142 in terrestrial samples has challenged this assumption as it requires the existence of a complementary reservoir with sub-solar system neodymium-142 to balance this excess [2]. The whereabouts of this reservoir remains unclear but has fundamental implications for the structure and composition of the Earth. It has either been (1) isolated in the Earth’s interior within 30 million years of the formation of the solar system [2] or (2) lost to space during the collision of planetary building blocks [1]. A radically different explanation would be (3) an Earth, which accumulated from material of non-bulk solar composition [1]. Solving this problem will have a decisive impact on our notion of planet formation. As neodymium isotope data alone cannot answer this question, the project will address the issue by using combined cerium and neodymium isotope data of selected terrestrial and extraterrestrial samples. Cerium-138 is the radiogenic decay product of the long-lived lanthanum-138 decay system. The process that formed the Earth will have caused a distinctive fractionation of the lanthanum/cerium ratio in these samples. This in turn will have led to indicative cerium-138 isotope signatures that will allow discrimination between the three scenarios in question.