Core formation, Hadean mattes and the timescale of Earth accretion

根据陨石中最古老的粒子的放射性年代测定，行星似乎在4567万年前开始形成，来自年轻太阳周围的尘埃和气体云。我们目前正处于一个非常有趣和令人兴奋的时刻，对于那些试图了解地球和其他行星是如何从这种原始的太阳系物质中成长起来的科学家来说。最近人们发现，在太阳系形成之初就存在着一些寿命很短的放射性同位素，它们可能是由附近的一颗爆炸的星星注入尘埃云的。这些放射性同位素现在完全衰变（“灭绝”），但它们的稳定衰变产物（“子体”）可以在陨石和地球，月球和火星中找到。由于已灭绝的同位素和它们的子体具有不同的化学性质，它们可能被用来研究倾向于分离它们的早期过程。例如，铪-182（已灭绝）衰变为钨-182，半衰期为900万年。钨倾向于进入行星的金属核心，而铪则留在它们的外部“岩石”部分。通过测量陨石和行星中子钨的同位素，已经表明一些小行星在太阳系开始的1-2百万年内形成了金属核心，而火星上的相同过程大约需要1200万年，地球上需要3000万年。该项目的目的是研究这些已灭绝的同位素及其子体的化学行为，以便我们更好地了解地球生长过程中发生的过程。一个主要的问题是，在太阳系形成后大约5000万年形成月球的巨大撞击发生时，地球是否失去了很大一部分很容易变成气体（“挥发性”）的元素。另一个是上面讨论的铪-182、钨-182系统测量的地球核心分离时间与铀-铅测量的时间之间的明显差异。铅具有一定的挥发性，因此有可能铅同位素测得的年龄（太阳系开始后的5000 - 1亿年）实际上是挥发损失的年龄，而不是核心形成的年龄。我们将能够区分这些可能性，通过确定不同挥发性的母同位素和子同位素在核心形成阶段的化学行为。为了做到这一点，我建议工作的分布铅（女儿的铀和母亲的铊），铊（女儿的铅-205）和银（女儿的钯-107）在金属核心偏析在高压，高温条件下的增长地球。由于这三个子元素的挥发性差别很大（铊最多，银最少），因此，我们可以将芯分离和挥发损失这两个过程分开。

Based on radioactive dating of the oldest particles in meteorites it appears that the planets began to form 4567 Million years ago from a cloud of dust and gas surrounding the young sun. We are currently at a very interesting and exciting time for those scientists trying to understand how the Earth and the other planets grew from this primitive solar system material. It has recently been discovered that there were a number of short-lived radioactive isotopes which were present at the beginning of the solar system, possibly injected into the dust cloud by a nearby exploding star. These radioactive isotopes are now completely decayed ('extinct') but their stable decay products ('daughters') can be found in meteorites and in the Earth, moon and Mars. Because the extinct isotopes and their daughters have different chemical properties, they may be used to investigate early processes which tended to separate them. For example, hafnium-182 (extinct) decays to tungsten-182 with a half-life of 9 million years. Tungsten tends to enter the metal cores of planets while hafnium remains in their outer 'rocky' parts. From measuring the isotopes of daughter tungsten in meteorites and the planets it has been shown that some asteroids formed metal cores within 1-2 million years of the beginning of the solar system while the same processes took about 12 million years on Mars and 30 million years on Earth. The aim of this project is to investigate the chemical behaviour of a number of these extinct isotopes and their daughters so that we may better understand the processes which took place as the Earth grew. One major question is whether or not the Earth lost a substantial fraction of those elements which could easily be turned into gases ('volatile') at the time of the giant impact which appears to have formed the moon about 50 million years after the beginning of the solar system. Another is the apparent discrepancy between the time of core separation on Earth measured by the hafnium-182, tungsten-182 system discussed above and that measured by uranium-lead. Lead is somewhat volatile, so it is possible that the age measured by lead isotopes (50-100 million years after the beginning of the solar system) is actually the age of volatile loss rather than the age of core formation. We will be able to distinguish between these possibilities by determining the chemical behaviour of parent and daughter isotopes of different volatility during the core formation stage. In order to do this I propose to work on distribution of lead (daughter of uranium and parent of thallium), thallium (daughter of lead-205) and silver (daughter of palladium-107) during metal core segregation under the high pressure, high temperature conditions relevant to the growing Earth. Because the three daughter elements have quite different volatilities (thallium most, silver least) the results will enable us to separate the two processes of core separation and volatile loss.