The mineralogic origins of magnetic remanence in silicate-hosted magnetite inclusions.

摘要

The seminal achievement of the paleomagnetic community over the last half-century was the successful reconstruction of 300 million years of reversal history of the Earth's magnetic field. This paleomagnetic timescale continues to be helpful to geophysicists studying plate tectonics, paleontologists studying evolution, and stratigraphers studying basin development. The reason this timescale covers only 1/15th of the Earth's past is because typical magnetic recorders, such as basalt and pelagic limestone, become progressively more rare and mineralogically altered with age. Pre-Mesozoic supracrustal rocks that have eluded alteration are not numerous enough to fully describe the magnetic field behavior during the three billion years encompassed within ancient geologic terrains.; This dissertation aims to lay the foundation for a new rock magnetic recorder that will allow researchers to investigate the behavior of the early Earth's geomagnetic field. Magnetic inclusions in silicates are the products of subsolidus precipitation (exsolution) during initial cooling and are capable of recording stable magnetizations consistent with expected geomagnetic field orientations. They are often found in the common igneous minerals clinopyroxene, plagioclase, orthopyroxene, amphibole, olivine and apatite, and in the metamorphic minerals sillimanite and saphirrine. Because the magnetic inclusions are isolated within their silicate hosts, they have the advantage of being mineralogically stable in the face of variations in temperature, pressure, chemical environment, and redox state caused by common geologic processes such as metamorphism, weathering, and hydrothermal alteration. Additionally, minerals such as clinopyroxene and plagioclase are major constituents of gabbros and anorthosites, which are commonly preserved in Precambrian shield areas.; The inclusions acquire their magnetizations as they pass through their blocking temperatures (530-580°C). Some inclusions experience a second unmixing event, subdividing the original magnetic inclusion into thousands of smaller magnetite blocks separated by ulvospinel lamellae. These nanometer-scale mineral structures create an unusually stable assemblage of magnetically interacting particles.; The ubiquity of these magnetic inclusions in mafic intrusive rocks indicates their potential utility in studying the Pre-Mesozoic magnetic field and elucidating early plate tectonic crustal motions. In addition, their presence in within the pyroxenes and plagioclases of oceanic gabbros supports the probability that they are significant contributors to seafloor magnetism.