| 英文摘要: | The first two billion years of terrestrial history arguably constitute the most important period of Earth's existence. Processes acting during this period effectively determined how this initially hot and inhospitable place in the universe became our present life-accommodating world. Terrestrial history is conventionally deciphered from the geological record. Due to the dynamic nature of Earth, its surface has been periodically rejuvenated from the time the planet was born. As a result, the isotopic and chemical heterogeneities observed in modern mantle-derived rocks primarily reflect processes associated with production and recycling of continental and oceanic crust through geological time; these likely bear no relation to primordial mantle differentiation processes. It is the early geological record, particularly between 4.5 and 2.5 Ga, from which information pertaining to the origin and early evolution of the Earth is most likely to be gleaned. Isotopic signatures created in early silicate reservoirs via radioactive decay of short- and long-lived nuclides have been sampled by early terrestrial magmas, such as komatiites, and preserved in early crustal rocks. These isotopic signatures can be used to constrain the timing of formation and the nature of these, now likely vanished, reservoirs. They can also be used to decipher the history of early planetary differentiation to the point in Earth history when crust formation and recycling processes, similar to those occurring today, took over as the main driver of chemical differentiation of the Earth?s interior.
This research project is aimed at constraining the origin and evolution of major silicate reservoirs in the early Earth. The primary goals of the proposed research are: (1) to assess whether magma ocean processes that likely occurred within the first 100 Ma of Earth history are recorded in the potentially deep mantle sources of komatiite systems, (2) to further test the hypothesis of Maier et al. (2009) that there was a detectable transition in highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) abundances in komatiites from the early to late Archean, and (3) to synthesize the isotopic and HSE abundance data for the komatiite systems examined here and other well-studied komatiite systems, integrating these data into viable models for the origin and temporal evolution of the early terrestrial silicate reservoirs contributing to these komatiite systems, and evaluate mixing times of the Earth?s mantle and the timing of late accretion. In order to achieve these goals, the 146,147Sm-142,143Nd, Lu-Hf, Pt-Re-Os, and 182Hf-182W isotope systematics and the lithophile trace and HSE abundances in five komatiite systems from around the globe, spanning a time interval of ca. 3.5 Ga in Earth history, will be characterized using state-of-the-art analytical techniques and interpreted. This study will help clarify the earliest history of Earth, and, therefore, will have relevance to the long-debated question of how planets form and evolve, and may ultimately improve our understanding of the modern Earth. Some of the expected scientific advances will be the result of involvement of up to three undergraduate students as part of their required senior research thesis work, thus, providing valuable training for the future careers of these budding scientists. Support for this research will help sustain the Isotope Geochemistry Laboratory?s mission to share and collaborate world-wide. The results of the proposed work will be published in peer-reviewed scientific journals, presented at professional conferences and invited lectures, and discussed at Departmental seminars. The outreach efforts will also include collaborations between scientists from three continents. |