Archean paleointensity from layered intrusions

Layered intrusions are a source for reliable measurements of the ancient geomagnetic field. Their magnetic properties are also an indirect record of crystal accumulation and postcumulus processes. This dissertation examines these two separate but related issues.; Chapters 1, 2, and 3 focus on changes in geomagnetic field intensity over long time scales appropriate to changes in core-mantle boundary conditions, the Earth's thermal state, and the development of the inner core. Chapter 1 assesses trends in paleofield intensity (paleointensity) using published estimates and new measurements on submarine basaltic glass. Based on the new measurements and reliable paleointensities in the existing database, the Earth's average dipole moment average for the past 300 million years was approximately 5 × 1022 Am2. No long-term trends in the distribution of dipole moments are obvious in the Chapter 1 dataset.; Chapter 2 examines the effects of remanence anisotropy and cooling rate on paleointensity experiments. These effects are particularly important in paleointensity studies on rocks from large layered intrusions like the late Archean Stillwater Complex in Montana. We present a technique to correct paleointensity estimates for the effects of remanence anisotropy, and examine a cooling rate correction as well.; Chapter 3 presents a large set of paleointensity measurements from the Stillwater Complex. The record a late Archean field of 29.2 ± 9.1 μT (corrected for cooling rate and anisotropy), which corresponds to a dipole moment (5.1 × 1022 Am2) close to the Earth's average for the last 300 Ma. The distribution of paleointensities is also consistent with more recent field variability.; Chapters 4 and 5 investigate the relationship between magnetic and silicate fabrics. Chapter 4 contains magnetic anisotropy measurements from single crystals of orthopyroxene, clinopyroxene and plagioclase (and their inclusions). These measurements are used to interpret whole-rock magnetic anisotropy measurements from the Stillwater Complex. Chapter 5 proposes a new technique for processing silicate petrofabric data. Crystallographic fabrics obtained using the new technique are compared to petrofabrics obtained using other techniques, as well as to magnetic anisotropy measurements.