Histories of magnetic mineral diagenesis, geodynamics and geomagnetic paleointensity as recorded by rock- and paleomagnetism.

摘要

Rock magnetic and paleomagnetic approaches are used to investigate a range of geological and geophysical issues, including the nature of diagenesis in pelagic sediments, techniques of low-temperature magnetometry, the origin of flood basalts, and the evolution of the early Earth's core.; In Chapters 1 and 2 (published as two papers in Journal of Geophysical Research) the fidelity of pelagic sediments as paleomagnetic and paleoenvironmental recorders is studied. The processes of maghemitization and magnetic reduction diagenesis are shown to play dominant roles in controlling the low-temperature magnetic properties of the sediments. A technique, based on the time-dependency of coercivity of remanence, is developed to acquire quantitative estimates of superparamagnetism as a proxy for paleoenvironmental and biogeochemical processes in the sediments.; In Chapter 3 (published in Earth and Planetary Science Letters), magnetic properties of cation-deficient and stoichiometric magnetite at very low temperatures (120K) are studied. The discovery of several low temperature magnetic phenomena, including a field memory effect is discussed. A theoretical interpretation of these observations is also provided in terms of interaction between magnetic and twin domains in magnetite. Chapter 4 (submitted to EOS) describes the field memory effect for a general scientific audience.; Chapter 5 addresses the problem of the origin the Siberian Traps. Paleomagnetic Euler pole analysis is used to reconstruct the position of the Traps to the time of their eruption at ∼250 million years ago. The results support a hotspot origin of the Traps and further constrain the long-lived nature of plume sources in the mantle currently underlying the North Atlantic/Arctic region.; Chapter 6 concern the intensity of the Earth's magnetic field at the Archean/Proterozoic boundary as recorded by single plagioclase crystals from dikes of northern Karelia (Russia). The preliminary paleointensity estimate, which is similar to the modern field strength, suggest the inner core started to grow by ∼2.45 Ga.; Chapter 7 describes new rock magnetic and paleointensity results from submarine basaltic glass, subaerial basaltic glass and closely associated volcanic rocks from Ocean Drilling Program Leg 197 (Emperor Seamounts of the northwestern Pacific Ocean).