Speleothems are secondary cave formations of calcium carbonate crystallites and usually contain a small fraction of noncarbonate materials. Iron oxide minerals in this noncarbonate fraction are capable of recording the direction of the Earth's magnetic field at the time they were encapsulated by the growing carbonate and may capture a nearly continuous recording of ancient geomagnetic field behavior. Further, the nature of the magnetic mineral assemblage (e.g., its constituent compositions, concentrations, and grain size distributions) provides a potential link to the environmental conditions present above a cavern. Thus, the dissolution of a speleothem and magnetic extraction of its iron oxide minerals followed by complementary rock magnetic and electron microscopic analyses can reveal fine details about Earth processes at the time of formation. However, trace concentrations of strongly magnetic minerals such as magnetite often mask the presence of weakly magnetic minerals, including goethite and hematite. To address these challenges, we developed a simple flask method for the efficient magnetic extraction and separation of weakly magnetic minerals using a simulated mineral assemblage and a natural speleothem sample. Strongly magnetic minerals were extracted with greater than 90% efficiency by mass. Additionally, by extracting the majority of strongly magnetic minerals, weakly magnetic minerals in the remaining mineral residue were successfully analyzed by low-temperature magnetic characterization and XRD. When compared to existing extraction methods, the flask extraction method recovered a consistently higher percentage of strongly magnetic minerals while avoiding less magnetic minerals. Furthermore, the flask extraction method requires considerably less speleothem to recover a representative amount of magnetic material for characterization than other methods, resulting in less destruction of the speleothem. This improved technique advances the future of speleothem magnetism as a practical tool for resolving Earth's paleomagnetic and environmental history.

Simple and efficient separation of magnetic minerals from speleothems

248th National Meeting (2014)

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