| 英文摘要: | The research project will document variations of the Earth's magnetic field strength for the period from approximately 1440 to 1380 million years ago to test models of geomagnetic field formation and the thermal evolution of the Earth. The Earth's magnetic field protects the Earth's surface from harmful cosmic radiation and potential interruptions of communication systems and electrical grids. To predict the stability and strength of the Earth's magnetic field, it is essential to understand how the field formed and what processes control its strength. It has been argued that the nucleation of the solid inner core led to a prominent and robust increase in geomagnetic strength, but the timing of inner core formation is in some dispute. Sparse and possibly biased data on the intensity of the magnetic field have been used to argue for inner core formation at about 1400 million years ago, but recent estimates of core thermal conductivity suggest that inner core growth may have begun only in the latest Precambrian, approximately 600 million years ago. The research in this project will provide high-quality, well-dated estimates of the ancient magnetic field over a critical time period in the evolution of Earth's magnetic field.
Integrated magnetic, petrologic and thermochronologic data from the well-dated 1430 million year old Laramie anorthosite complex will be used to construct a time series of geomagnetic intensity (and directional) variations. Samples will be collected from a series of eight drill cores, each about 50m in length, and complementary surface coring systematically away from the preserved cooling margin of the intrusion. The paleointensity study will focus on small subsamples designed to isolate the high-stability remanence carried by magnetite within plagioclase crystals. The precise cooling history will be determined from U-Pb radioisotopic dates of the mineral apatite, which has a thermal closure temperature of approximately 450°C, close to the minimum temperatures of remanence acquisition. The pristine cores will be dual purposed as they will also be used to investigate primary igneous petro-fabrics and the processes by which igneous foliations and any lineations have been produced, thereby deepening our understanding of the crystallization processes of igneous bodies. The project will train several undergraduate students, form the basis for one PhD dissertation, and strengthen collaboration among scientists at two institutes, the University of California San Diego (Scripps Institution of Oceanography) and the University of Wyoming (Department of Geology and Geophysics). |