Core formation, Hadean mattes and the timescale of Earth accretion

根据对陨石中最古老粒子的放射性测年，这些行星似乎是在4.567亿年前由年轻太阳周围的尘埃和气体云形成的。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生长出来的科学家来说，我们目前正处于一个非常有趣和令人兴奋的时刻。最近人们发现，在太阳系形成之初，有许多短暂的放射性同位素存在，可能是由附近一颗正在爆炸的恒星注入尘埃云中的。这些放射性同位素现在已经完全衰变(“绝迹”)，但它们稳定的衰变产物(“子体”)可以在陨石以及地球、月球和火星中找到。由于已灭绝的同位素及其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，Hf-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，而氦则留在行星外部的“岩石”部分。通过测量子钨在陨石和行星中的同位素，已经表明，一些小行星在太阳系开始的100-200万年内形成了金属核心，而同样的过程在火星上需要大约1200万年，在地球上大约需要3000万年。这个项目的目的是调查一些已经灭绝的同位素及其子体的化学行为，以便我们可以更好地了解随着地球的成长而发生的过程。一个主要的问题是，在大撞击的时候，地球是否损失了很大一部分元素，这些元素很容易被转化为气体(挥发性)。大撞击似乎是在太阳系开始大约5000万年后形成了月球。另一个是上面讨论的Hf-182，W-182系统测量的地球核分离时间与铀-铅系统测量的地球核分离时间之间的明显差异。铅具有一定的挥发性，因此用铅同位素测量的年龄(太阳系开始后的5,000万-1亿年)实际上可能是挥发性损失的年龄，而不是核心形成的年龄。我们将能够通过确定不同挥发性的母体和子体同位素在岩心形成阶段的化学行为来区分这些可能性。为了做到这一点，我建议研究在与地球生长相关的高压、高温条件下，金属核分离过程中铅(铀的子体和铊的母体)、铊(铅-205的子体)和银(钯-107的子体)的分布。由于三个子元素的挥发度有很大的不同(最多的是铊，最少的是银)，这一结果将使我们能够将芯分离和挥发损失两个过程分开。

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.