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的变化，但玻璃中剩磁的来源仍然知之甚少，这导致一些人质疑从玻璃估计古磁场强度的有效性。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--也就是说，当玻璃最初冷却时，磁化必须在高温下锁定。如果认为磁化作用是低温化学剩磁的说法是正确的，那么玄武岩玻璃对古地磁场的估计可能是无效的。这项研究旨在阐明玄武岩玻璃中(钛)磁铁矿的形成过程，以及它形成的条件和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、冷却速度、成分)和已知磁场中合成的。起始原料要么是粉状的天然拉斑玄武岩，要么是由混合试剂级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和分析透射电子显微镜，可以探索磁铁矿在玻璃中沉淀和生长的条件和过程。这些结果不仅对玻璃中磁铁矿的来源具有指导意义，而且对其在重新加热古强度实验中的稳定性也具有指导意义。显微分析技术还可以对颗粒大小和成分进行表征，同时对合成玻璃样品和不同年龄的天然玻璃样品使用一套岩石磁性技术进行磁性表征。通过这种方式，合成玻璃的磁性矿物学和磁性很容易与天然玻璃的磁性进行比较。最后，古强度实验提供了对合成玻璃样品准确记录环境场强的能力的直接测试。

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.