Observations under electron microscopy of magnetic minerals extracted from speleothems

Magnetic minerals have been extracted from a selection of British speleothems and studied by electron microscopy and energy dispersive X-ray analysis (EDXA). Observations of the surface morphology, size and elemental composition of these grains has assisted in determining their origin. Thereby, direct evidence as to the carriers of the Natural Remanent Magnetization (NRM) of speleothems is provided. Three groups of grain morphology were identified: (1) detrital grains ranging in size from 0.01 μm to ⪢ 10 μm, composed of magnetite, hematite and titanomagnetite; (2) unabraded grains ranging in size from 0.01 μm to 0.1 μm, composed of magnetite, these grains were further subdivided into those having an affinity with bacterial magnetites and those having an affinity with inorganic authigenic magnetites; (3) needle-like grains, less than 2 μm in length, probably composed of goethite. Preliminary laboratory experiments suggest that authigenic magnetite and goethite may precipitate under conditions likely to prevail during speleothem growth. The presence of detrital magnetic grains in all samples studied is strong evidence for the presence of a Detrital Remanent Magnetization (DRM) in all speleothems. The presence of authigenic magnetite in some samples is the first direct evidence for the presence of a Chemical Remanent Magnetization (CRM) in some speleothems.