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

假设地球的化学成分与整个太阳系相似，这是我们理解地球如何形成的关键[1-3]。最近在陆地样本中检测到的超太阳系Nd142挑战了这一假设，因为它需要存在一个与次太阳系Nd142互补的水库来平衡这种过剩[2]。这个水库的下落仍然不清楚，但对地球的结构和组成有根本的影响。它要么(1)在太阳系形成后3000万年内被隔离在地球内部[2]，要么(2)在行星积木[1]的碰撞中消失在太空中。一种完全不同的解释是(3)地球，它是由非大宗太阳成分物质积累而成的[1]。解决这个问题将对我们关于行星形成的概念产生决定性的影响。由于单靠钕同位素数据不能回答这一问题，该项目将通过使用选定的陆地和地外样品的Ce和Nd同位素组合数据来解决这一问题。Ce-138是长寿命镧-138衰变系统的放射性衰变产物。形成地球的过程将导致这些样品中La/Ce比率的明显分馏。这反过来将导致指示性的Ce-138同位素签名，这将允许在所讨论的三种情况之间进行歧视。

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.