Collaborative Research: Origin of Magnetite and Magnetic Remanence in Submarine Basaltic Glass and Implications for Glass Paleointensities

海底玄武岩玻璃越来越多地被用于研究地磁场强度变化，但玻璃中剩磁的起源仍然知之甚少，导致一些人质疑玻璃古强度估计的有效性。为了恢复有效的古强度估计，玻璃中的剩磁必须是主要的热剩磁-即磁化必须在玻璃最初冷却时的高温下锁定。 如果磁化是低温化学剩磁的说法是正确的，那么玄武岩玻璃的古磁场估计可能是无效的。 本研究旨在阐明（钛）磁铁矿在玄武岩玻璃中形成的过程，以及它形成的条件和它作为地磁场变化记录器的保真度。 玻璃是在一系列条件（氧逸度，冷却速率，组成）和已知的磁场下合成的。 起始材料是粉末状天然拉斑玄武岩或通过混合试剂质量组分氧化物制备的简化合成组合物。再加热氧化扩散实验，结合卢瑟福背散射光谱法和分析透射电子显微镜，允许探索的条件下和磁铁矿沉淀和生长在玻璃中的过程的领域。这一结果不仅对玻璃中磁铁矿的成因具有指导意义，而且对玻璃中磁铁矿在再加热古强度实验中的稳定性也具有指导意义。 微量分析技术还允许表征粒度和成分，这是补充磁性表征使用一套岩石磁性技术的合成玻璃样品，以及不同年龄的天然玻璃样品。通过这种方式，合成玻璃的所得磁性矿物学和磁性性质容易与天然玻璃的磁性矿物学和磁性性质进行比较。最后，古强度实验提供了一个直接测试的能力，合成玻璃样品准确地记录环境场强度。

Submarine basaltic glass is increasingly being used in studies of geomagnetic field intensity variation, yet the origin of the magnetic remanence in glass remains poorly understood, leading some to question the validity of paleointensity estimates from glass. To recover valid paleointensity estimates, the magnetic remanence in the glass must be a primary thermoremanence - i.e. magnetization must be locked in at high temperatures when the glass initially cools. If suggestions that the magnetization is instead a low-temperature chemical remanence are true, then paleofield estimates from basaltic glass might be invalid. This study is designed to elucidate the process by which (titano)magnetite forms in basaltic glass, as well as the conditions under which it forms and its fidelity as a recorder of geomagnetic field variations. Glass is synthesized under a range of conditions (oxygen fugacity, cooling rate, composition) and in a known magnetic field. The starting materials are either powdered natural tholeiitic basalt or simplified synthetic compositions prepared by mixing reagent-quality component oxides. Reheating oxidation diffusion experiments, combined with Rutherford backscattering spectrometry and analytical transmission electron microscopy, allow an exploration of the field of conditions under which and the process by which magnetite precipitates and grows in glass. The results have direction implications not only for the origin of magnetite in glass, but also for its stability during reheating paleointensity experiments. The microanalytical techniques also allow characterization of grain size and composition, which are complimented by magnetic characterization using a suite of rock magnetic techniques on both the synthetic glass samples, as well as natural glass samples of varying ages. In this way the resulting magnetic mineralogy and magnetic properties of the synthesized glasses are readily compared to those of natural glasses. Finally, paleointensity experiments provide a direct test of the ability of the synthetic glass samples to accurately record the ambient field intensity.