The Origin and Survival of Chemical Heterogeneities in the Earth's Mantle

准确地确定地球的化学成分一直是并仍然是地球科学中最根本的挑战之一。它不仅对类地行星如何形成和演化这一长期争论的问题具有重要影响，而且对于我们理解正在进行的过程对我们星球的影响也具有重要意义。地幔是所有陆地储集层中最大的；它占地球体积的83%，延伸到地球表面以下2900公里。由于先前对陆地岩石记录的开创性研究，这一巨大储集层的化学和同位素不均一性质早已得到证实。一些不均质性被认为是原始的，反映了最初的行星吸积/分异和岩浆海洋结晶过程。另一些则可能是漫长而猛烈的陆地吸积史的结果，或者是与地球动态制度，特别是地壳循环有关的后期过程的结果。尽管进行了长期的、协调一致的研究，但早期地幔化学和同位素不均一性的性质、起源、规模和寿命仍然不清楚。该项目旨在通过将精心挑选的一套最先进的现代分析工具应用于西澳大利亚独特的35亿至27亿年前的科马提石样本，来填补我们对这些基本问题的理解方面的现有空白。这些样品特别有利于研究地球历史前20亿年内发生的过程，在地壳形成和再循环过程(与今天发生的过程类似)成为地球内部化学分化的主要驱动力并抹去了地球年轻繁荣的所有证据之前。这项研究旨在限制早期地幔中化学和同位素不均一的起源和时间演化。主要目标是：(1)评估西澳大利亚3.52至2.69 Ga科马提岩地幔来源中记录的化学/同位素不均一性的大小，(2)确定哪种基本的早期地球过程，如晚期增生、岩浆海洋结晶、原始地壳提取和地壳再循环，造成了这些不均一性，以及(3)将这些岩石的新的短期和长期同位素和微量元素丰度数据与太古代科马提岩系统的现有数据合并成地球地幔中全球化学/同位素不均一性的起源和时间演化模型。为了实现这些目标，该小组将利用热电离质谱学和电感耦合等离子体质谱技术研究3.5至2.7Ga科马提岩系统的钻石钻芯样品中的同位素系统学、亲石微量元素和高度亲铁元素丰度。这些科马提质岩系来自西澳大利亚的古代皮尔巴拉和伊尔加恩克拉通，其形成时间跨度约为地球历史的800 Ma。选择这些地点是为了覆盖空间上相关的太古宙克拉通内最大程度的时间变化，因为选定的样品按照太古宙标准保存得很好，预计将提供了解早期地幔的窗口，而且现有数据有限，表明这些科马提岩系统记录了非常早期的地幔分化过程。预计结果将使我们能够评估原始岩浆海洋的分化、晚期吸积和地壳再循环的可能影响，并允许在地球历史的前半部分对地幔混合时间施加新的限制。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Accurately determining the Earth's chemical composition has been and remains one of the most fundamental challenges in Earth sciences. It has important implications not only for the long-debated question of how terrestrial planets form and evolve, but also for our understanding of the consequences of the ongoing processes on our planet. The Earth's mantle is the largest of all terrestrial reservoirs; it constitutes 83% of the Earth's volume and extends to 2900 km below its surface. The chemically and isotopically heterogeneous nature of this vast reservoir has long been established as a result of previous pioneering studies of the terrestrial rock record. Some of the heterogeneities have been argued to be primordial, reflecting initial planetary accretion/differentiation and magma ocean crystallization processes. Others were likely created as a result of a protracted, violent terrestrial accretion history, or to have resulted from later processes associated with the dynamic regime of the planet, especially crustal recycling. Despite the long-term, concerted research effort, the nature, origin, scale, and longevity of early mantle chemical and isotope heterogeneities are still not well understood. This project is aimed at filling the existing gaps in our understanding of these fundamental issues by applying a carefully chosen, comprehensive set of state-of-the-art modern analytical tools to a collection of unique 3.5 to 2.7 billion year old komatiite samples from western Australia. These samples are especially advantageous to study processes that occurred within the first 2 billion years of Earth history, before the crust formation and recycling processes, similar to those occurring today, took over as the main driver of chemical differentiation of the Earth's interior and erased all the evidence of Earth's youthful exuberance.This research effort is aimed at constraining the origin and temporal evolution of chemical and isotopic heterogeneities in the early Earth's mantle. The primary goals are to: (1) assess the magnitude of chemical/isotopic heterogeneities recorded in the mantle sources of 3.52 to 2.69 Ga komatiites from Western Australia, (2) establish which of such fundamental early Earth processes, as late accretion, magma ocean crystallization, primordial crust extraction, and crustal recycling, were responsible for creating these heterogeneities, and (3) integrate the new short- and long-lived isotopic and trace element abundance data for these rocks with the existing data for Archean komatiite systems into models for the origin and temporal evolution of global chemical/isotope heterogeneities in the Earth's mantle. In order to achieve these goals, the team will study isotope systematics, and lithophile trace element and highly siderophile element abundances in diamond drill core samples of the 3.5 to 2.7 Ga komatiite systems using the thermal ionization mass-spectrometry and inductively-coupled plasma mass-spectrometry techniques. These komatiitic systems are from the ancient Pilbara and Yilgarn Cratons of Western Australia, and their formation spans a time interval of ca. 800 Ma of Earth history. These localities were selected in order to cover the greatest possible extent of temporal variations within spatially related Archean cratons, because the selected samples are well preserved by Archean standards and are expected to provide a window into the early terrestrial mantle, and because limited existing data indicate that these komatiite systems record very early mantle differentiation processes. The results are expected to allow us to assess the possible effects of differentiation of a primordial magma ocean, late accretion, and crustal recycling, as well as permit new constraints to be placed on the mixing times of the mantle during the first half of Earth's history.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.