Tungsten and Ruthenium Isotopic Study of the Chemical Evolution of Earth

地球很可能是在太阳系历史的前5000万年里，越来越大的星子发生一系列碰撞的结果。在最后一次巨大的撞击导致地球的大部分融化和月球的形成后，地球的生长基本上停止了。令人惊讶的是，在古代和年轻岩石中某些元素的同位素组成中，有强有力的证据表明，地界是在地球内部形成的。在大碰撞之前的地球初始构造时期，这些区域形成了地幔，并且以某种方式保存到了现在。这项研究将检查来自这些区域的岩石的钨和钌同位素组成，为地球成长过程中发生的过程提供新的见解，并有助于限制我们星球构建块的遗传起源。陆地岩石中182-钨丰度的微小变化可能反映了182-铪的衰变，再加上地球硅酸盐部分中这两种元素非常早期的化学分馏。先前的研究表明，在一些原始岩石中，182-钨的适度变化可能反映了早期形成的岩浆海洋中发生的过程。这项研究将寻求更全面地确定同位素非均质性的原因，并评估为什么地幔的对流搅拌在超过45亿年的时间里没有将非均质性混合掉。钌的同位素组成在我们太阳系的行星体中是不同的，这是核合成成分的不同混合物的结果。因此，如果地球是由遗传多样性的物质构成的，那么通过钨同位素变化确定的早期形成的地幔域可能含有与今天地幔主要成分同位素上不同的钌。如果是这样的话，这个特定的组成部分可能与一种特定类型的宇宙化学前体有关。

The Earth was likely built as a result of a series of a series of collisions of increasingly larger planetesimals, during the first 50 million years of Solar System history. Growth of the planet largely terminated after a final, giant impact that led to melting of much of the Earth and the creation of the Moon. Surprisingly, there is strong evidence present in the isotopic compositions of some elements in both ancient and young rocks that domains were created within the Earth?s mantle during the primary construction period of the Earth before the giant impact, and these domains have been somehow preserved to the present. This study will examine the tungsten and ruthenium isotopic compositions of rocks derived from these domains to provide new insights to processes occurring within the Earth as it grew, as well as help to constrain the genetic origins of the building blocks of our planet. Small variations in the abundance of 182-tungsten in terrestrial rocks likely reflects the decay of 182-hafnium, coupled with very early chemical fractionations of the two elements within the silicate portion of the Earth. Prior studies have suggested that modest variations in 182-tungsten among some primitive rocks may reflect processes occurring in early-formed magma oceans. This study will seek to more fully establish the causes of the isotopic heterogeneities, and assess why convective stirring of the mantle has not mixed away the heterogeneities in more than 4.5 billion years. The isotopic composition of Ru varies among planetary bodies in our Solar System as a result of the incorporation of diverse mixtures of nucleosynthetic components. Hence, if the Earth was built from genetically diverse materials, early-formed mantle domains identified by tungsten isotopic variations, may contain ruthenium that is isotopically distinguishable from the dominant composition of the mantle today. If so, the particular building block identified may be tied to a specific type of cosmochemical precursor.